Rockbox is a Linux-based replacement firmware for DAPs (digital audio players). It is written in the C programming language (and some bits of assembler).

http://www.rockbox.org/

Rockbox currently runs on many mainstream DAPS (Archos, iPod, iRiver, etc.), with various degrees of functionality depending on the hardware characteristics of the target devices (e.g. color LCD screen, audio recording capability, FM-radio, remote control, scroll-wheel etc.). See:

http://build.rockbox.org/

http://www.rockbox.org/twiki/bin/view/Main/TargetStatus

http://www.rockbox.org/twiki/bin/view/Main/OtherTargets

Rockbox is GPL-licensed open-source software (currently GPL 2.0, but there are currently some talks regarding a possible move to GPL 3.0 or "GPL 2.0 and later"). See:

http://www.rockbox.org/mail/archive/rockbox-dev-archive-2007-09/0020.shtml

The main functionality of Rockbox is extended audio playback (mp3, Ogg-Vorbis, Flac, etc.) and easy-to-use navigation features (id3 tag browser, bookmarks, playlists, etc.). The main design goal is to provide a lightweight, fast-loading firmware that can boot on as many devices as possible. It is not to be an all-rounder PDA (Personal Digital Assistant). See:

http://www.rockbox.org/twiki/bin/view/Main/WhyRockbox

However, this does not mean that Rockbox is minimalist: in fact it can be fully customized by users (graphical themes, language) and extended by programmers (core and plugins).

The default Rockbox software stack includes many additional plugins, such as: graphic demos, visualizations, calendar, games (gameboy emulation, Doom ;) ), audio utilities, jpeg viewer, and even an mpeg video player ! See:

http://www.rockbox.org/twiki/bin/view/Main/PluginIndex

Rockbox comes with a built-in self-voicing user-interface, which makes its functionality accessible to blind users or those with visual impairments (DAP screens are usually very small). Because this category of users consume a lot of information in audio form, such functionality is a much welcome addition on a DAP. See:

http://www.rockbox.org/twiki/bin/view/Main/BlindUsersIndex

http://www.rockbox.org/twiki/bin/view/Main/BlindFAQ

The voices used to narrate the user-interface are pre-generated using TTS (Text-To-Speech) on a desktop computer. See:

http://www.rockbox.org/twiki/bin/view/Main/VoiceHowto

http://www.rockbox.org/twiki/bin/view/Main/VoiceFiles

http://www.rockbox.org/twiki/bin/view/Main/VoiceBuilding

There are plans for integrating a TTS engine inside the Rockbox firmware. See:

http://www.rockbox.org/twiki/bin/view/Main/TextToSpeech

There is in fact a working port of eSpeak, but it's not officially supported due to licensing issues (GPL 2.0 vs 3.0). See:

http://www.rockbox.org/tracker/task/7660?histring=tts