Rockbox
Rockbox is a GPL-compliant open source operating system for MP3 music players (portable digital audio players, or DAPs). The Rockbox Project began in 2002 and was first implemented on the Archos Studio DAP because of owner frustration with severe limitations in the manufacturer-supplied user interface and device operations.
Rockbox can completely replace the host device's operating system firmware and has matured to become an extensible, flexible platform that provides a plug-in architecture for adding PDA functionality, applications, utilities, and games, and has also managed to retrofit video playback functionality onto DAPs first released in mid-2000. Recently, Rockbox now includes a voice-driven user-interface suitable for operation by blind and visually impaired users.
Although Rockbox's official title is "Rockbox: Open Source Jukebox Firmware", in many instances it is not actually installed to (or run from) flash memory. Instead a minimal bootloader is installed in the supported device's flash which is capable of either loading Rockbox from the hard disk or, alternately, the original factory firmware.
Archos devices
Rockbox was first designed for the Archos series of MP3 players and player/recorders. These devices have relatively weak main CPUs and instead offload music playback to dedicated hardware MP3 decoding chips. Rockbox, therefore, was unable to significantly alter playback capabilities and instead greatly improved user interface and added plugin functionality not present in the factory firmware.
Rockbox is capable of being permanently flashed into flash memory on the Archos devices, making it a literal firmware replacement.
iRiver devices
Beginning in late 2004, an effort began to port Rockbox to the ColdFire-powered devices manufactured by iRiver, focusing on the H1xx series of hard drive players (H110/H120/H140). These devices perform audio decoding in software, allowing Rockbox to potentially support many more music formats than the original firmware as well as bringing the extensibility and increased functionality already present in the Archos ports. Rockbox is run from the hard disk on these devices, after being started with a custom bootloader.
As of November 2005, the iRiver port is in beta testing. Most major functions are fully supported including gapless playback, "on the fly" (OTF) playlists, FM radio, remote control and basic recording (including FM recording). Supported formats include: MPEG audio (MP3/MP2), Ogg Vorbis, Musepack, AC3 (DVD audio), AAC (experimental, not yet real-time), FLAC, Apple Lossless, WavPack and uncompressed WAV audio. Plugins include a greyscale JPEG viewer, the RockBoy Game Boy emulator (not yet real-time) and several games.
Ports are underway for the H3xx series and the iAudio X5 but both lag considerably behind the H1xx effort.