CyanogenMod
Description
"One open source project with a long pedigree – and an impressive user base – is CyanogenMod:
an aftermarket firmware for over forty cell phones and tablets based on the open-source Android operating system. It offers features not found in the official Android-based firmwares of vendors of these devices, including native theming support (also known as the "T-Mobile Theme Engine"), a codec for the Free Lossless Audio Codec (FLAC), compressed cache (compcache), a large APN list, an OpenVPN client, a reboot menu, support for Wi-Fi, Bluetooth, and USB tethering, toggles in the notification pull-down (such as Wi-Fi, Bluetooth, GPS, and many more), as well as other enhancements.
CyanogenMod was also the first mobile OS to incorporate BFS as the task scheduler, a change that has been merged into experimental branches in the official Android source tree. CyanogenMod claims to increase performance and reliability over official firmware releases.
As of 17 July 2011, CyanogenMod has been installed on over half a million devices." (http://www.h-online.com/open/features/Is-Android-s-bane-a-boon-for-Free-Software-1358433.html)