Compiled by Valentin Spirik:
Audacity http://audacity.sourceforge.net/ is "free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems." (*)
Streamripper http://streamripper.sourceforge.net/ is "an Open Source (GPL) application that lets you record streaming mp3 to your hard drive", e.g. lets users record webradio.
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."
The VLC media player (or VideoLAN) http://www.videolan.org/ "is a free cross-platform media player" that "supports a large number of multimedia formats, without the need for additional codecs" and it "can also be used as a streaming server".
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.
ccHost http://wiki.creativecommons.org/CcHost "is Creative Commons' open source (GPL licensed) project that powers ccMixter and is the winner of the Linux Journal LinuxWorldExpo Product Excellence Award for Best Open Source Solution."
mxchg http://mxchg.com/ is "Your Fresh New Music Centre" and also allows artists to "Distribute music to their fans directly. ... The Media Exchange is 'open source' (all the source code is included on the CD)".
Jokosher http://www.jokosher.org/ is "a simple yet powerful multi-track studio. With it you can create and record music, podcasts and more, all from an integrated simple environment." Jokosher is "in a pre-release stage". Linux only.
Songbird http://www.songbirdnest.com/ "is a Web player built from Firefox's browser engine. Songbird is open source, will run on Windows, Mac OS X and Linux and supports user contributed, cross-platform extensions." In development.
(*) 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)