Audio/Video Players/Aggregators, Web TV and Codec
The VLC media player (or VideoLAN) http://www.videolan.org/ is an open-source video (and audio) player available for all major platforms and capable of playing back almost all video formats found online. The VLC can be used as a streaming server http://www.videolan.org/streaming/ and also for transcoding video(streams). (*)
The MPlayer http://www.mplayerhq.hu/design7/news.html, open-source, is "the Movie Player for Linux", but there is also the MPlayer OS X http://mplayerosx.sourceforge.net/.
Aggregators, Web TV
Miro http://www.getmiro.com/, open-source, brings independent internet tv (and using the Broadcast Machine http://www.getmiro.com/create/broadcast/ anyone can publish media - also via Bittorrent). Available for all major platforms.
FireAnt http://getfireant.com/ is - like Miro or Apple's iTunes http://www.apple.com/itunes/ - a feed aggregator, meaning a user can subscribe to a particular channel via a publisher's RSS feed and the app will automatically download the latest episode of a particular programme. FireAnt is specially popular with parts of the videoblogging community, and it also offers one of the best ways of playing back downloaded .flv (Flash Video) files. (KeepVid http://keepvid.com/ lets users download Flash Videos from sites like YoutTube.) Available for Windows and Mac OS X.
PenguinTV http://penguintv.sourceforge.net/, open-source, is "not just another RSS feed reader. It is designed from the ground up to work seamlessly with podcasts and video blogs". Available for Linux.
Tribler https://www.tribler.org/, licensed under the LGPL (en.wikipedia.org) , "is a social community that facilitates filesharing through a so called peer-to-peer (p2p) network." Available for all major platforms.
Adobe Media Player http://labs.adobe.com/downloads/mediaplayer.html, commercial software, "is a prerelease" and "being made available for consumers to test the user experience, and for content publishers to test the compatibility of their existing Media RSS feeds with Flash-based video content." This might be of interest since the Flash Player 9 http://labs.adobe.com/technologies/flashplayer9/ now supports H.264. Available for Windows and Mac OS X.
FOSS Codecs For Online Video http://wiki.transmission.cc/index.php/FOSS_Codecs_For_Online_Video:_Usability_Uptake_and_Development_1.2 "Usability Uptake and Development 1.2"
Flip4Mac http://www.flip4mac.com/wmv_download.htm is a commercial but free (there is a pro/to buy version) that allows Mac users to watch (newer) .wmv encoded files either via the web browser or with the QuickTime Player. Since the latest version of the open-source VLC (0.8.6) - see above - now also plays back videos encoded with the newer Windows Media 9 codec (on the Mac), Flip4Mac might not be so important any more for some Mac users.
QTAmateur http://www.mikeash.com/?page=software/qtamateur/index.html is "a small, simple QuickTime video player. It can play any format that QuickTime can understand, handle fullscreen video playback, and export files to any format that QuickTime can write ...includes a Batch Export function which can convert any number of movies at a time to another format."
Perian http://perian.org/ is "a free, open source QuickTime® component that adds native support for many popular video formats" (like .avi and .flv). Requires OS X 10.4.7.
Xiph QuickTime Components http://www.xiph.org/quicktime/ is "the solution for Mac and Windows users who want to use Xiph formats in any QuickTime-based application, e.g. playing Ogg Vorbis in iTunes."
Ogg Vorbis http://www.vorbis.com/ (for audio) and Ogg Theora http://www.theora.org/ (for video) are open-source alternatives for media encoding, also interesting for commercial online publishing since they are patent free and no license fees will have to be paid.
(*) VLC 0.8.6 (December 2006): Windows Media 9 and Flash Video now also supported under OS X. The only major format not supported yet on any platform is the increasingly unpopular RealVideo.
Audio/Video/Image Editing and 3D
Audacity http://audacity.sourceforge.net/ is an open-source audio editor, available for all major platforms. (*)
Ardour http://www.ardour.org/ "is a digital audio workstation. You can use it to record, edit and mix multi-track audio. ... If you've been looking for a tool similar to ProTools, Nuendo, Cubase SX, Digital Performer, Samplitude or Sequoia, you might have found it." For Linux and OS X (no Windows version), source code available.
Inkskape http://www.inkscape.org/ is an open-source vector graphics editor, available for all major platforms.
The Gimp http://www.gimp.org/ is an open-source image manipulation programme, available for all major platforms. To make the transition easier for Photoshop users there is also the GimpShop http://plasticbugs.com/?page_id=294.
CinePaint (Film Gimp) http://www.cinepaint.org/ is an open-source programme for deep paint manipulation and image processing (e.g. motion picture frame-by-frame retouching). An upcoming update is expected to support all major platforms.
ffmpegX http://www.ffmpegx.com/ is a popular shareware programme for transcoding video on the Mac, maybe not a tool that is easy to use, but works well once configured to one's needs. (The open-source VLC media player http://www.videolan.org/ can also be used for transcoding video and is available for all major platforms.)
Jashaka http://www.jahshaka.org/ is "the worlds first OpenSource Realtime Editing and Effects System", available for all major platforms.
LiVES http://lives.sourceforge.net/ is "a Free, Open Source video editor *and* a VJ tool." It is "fully cross platform for Linux and many flavours of Unix (e.g. BSD, openMosix, IRIX, OSX/Darwin)."
Kdenlive http://kdenlive.org/ is "a non linear video editor for the KDE environment running on Linux. It is based on the MLT video framework which relies on the FFMPEG project".
Cinelera is a "non-linear video editor and compositor for Linux", there are two versions http://www.heroinewarrior.com/index.php3 and http://cvs.cinelerra.org/.
Ubuntu Studio http://ubuntustudio.org/ is "A multimedia creation flavor of Ubuntu" and "aimed at the GNU/Linux audio, video and graphic enthusiast as well as professional. We provide a suite of the best open-source applications available for multimedia creation. Completely free to use, modify and redistribute. Your only limitation is your imagination."
Blender http://www.blender.org/ is an open-source 3D modeling/animation programme, available for all major platforms.
See also the Wikipedia iMovie and Windows Movie Maker entries for basic (commercial) video editing software (en.wikipedia.org). A basic DV video editing software for Linux is the open-source Kino (www.kinodv.org).
(*) Mac OS X Audacity bug: if you hear no sound when watching .flv videos (e.g. YouTube) after using Audacity: Open Audio MIDI Setup (/Applications/Utilities/) and change the "Audio Output setting" from "96000.0Hz" back to "44100.0Hz" (see also Mac OS X: No audio from certain multimedia content). (docs.info.apple.com)
Tip: Flash videos like on YouTube are always downloaded to the browser's cache when watching them! You can either search for them (if they are not in the "Cache" folder include "invisible items" or similar to your search) or download them again using a service like KeepVid http://keepvid.com/. In any case the file's extension .flv has to be added. For playback use e.g. the VLC http://www.videolan.org/.
Audacity http://audacity.sourceforge.net/ for free audio capturing on OS X and Windows explained in this digg discussion (digg.com).
The open-source Streamripper http://streamripper.sourceforge.net/ lets users record e.g. webradio as .mp3 files. For OS X users: StreamRipperX http://streamripperx.sourceforge.net/ (the old StreamRipperX 1.0.5 still works under OS X 10.3.9, not only 10.2 as mentioned).
KeepVid http://keepvid.com/ or Video Downloader http://javimoya.com/blog/youtube_en.php let users download Flash Videos from sites like YoutTube.
PodTube (OS X) and iTube (Windows) are for YouTube to iPod. (www.epicempire.com) Note: If this link is not working also try The Amazing YouTube Tools Collection. (www.indianwebmaster.org)
Wiretap http://www.ambrosiasw.com/utilities/wiretap/ "allows you to record audio from any running applications, as well as from any microphone, line-in, headset...". Commercial software. (Free option via Audacity: see above)
Snapz Pro X http://www.ambrosiasw.com/utilities/snapzprox/ "allows you to effortlessly record anything on your screen, saving it as a QuickTime® movie or screenshot". Commercial software.
Copernicus http://danicsoft.com/projects/copernicus/ "doesn't just take screenshots... allows you to save directly to RAM, increasing movie framerates significantly for shorter movies". Requires OS X 10.4. Free commercial software.
Screen Mimic http://www.polarian.com/products/ScreenMimic.php "allows you to create screencasts (recordings of your Mac OS X desktop) that can be saved as Adobe Flash (SWF), Flash Video (FLV), or Quicktime (MOV) files". Requires OS X 10.4. Commercial software.
CamStudio http://www.camstudio.org/ is an open-source "Free Screen Recording Software". Download via http://sourceforge.net/projects/camstudio/.
VH Screen Capture Driver http://www.free-codecs.com/download/VH_Screen_Capture_Driver.htm "is a DirectShow filter, which emulates Video Capture card and captures screen activity." Freeware.
List of Video Recording software from Softpedia http://www.softpedia.com/catList/112,0,1,0,1.html. Mainly commercial software, some freeware.
SnagIt http://www.softwarecasa.com/pprod/212401dd.htm "All the screen capture and editing functionality you’ll ever need, in one simply powerful program." Commercial software. (Free option via Audacity: see above)
Camtasia Studio http://www.softwarecasa.com/pprod/212402dd.htm "Full-Motion Screen Recording Software". Commercial software.
recordMyDesktop http://recordmydesktop.sourceforge.net/ is an open-source software that "offers also the ability to record audio, along with video."
See also the related online article How-to: Rip DVDs with VLC. (www.therealcaffeine.com, 21.01.07)
The Amazing YouTube Tools Collection http://www.indianwebmaster.org/showthread.php?t=174 lists "YouTube third party tools which enhance your YouTube experience" (scroll down to see the tools collection).
Telephony (VoIP)/Video Calls
Gizmo http://www.gizmoproject.com/ is "A free phone for your computer", offers free voicemail and conference calls, calls can be recorded and it supports the open-source Asterisk (www.asterisk.org) PBX software. Gizmo currently has no video. Available for all major platforms.
Skype http://www.skype.com/ lets you "Free your conversations", offers conference calls for up to five participants and free video calls. Available for all major platforms.