RockBox is an open source replacement firmware for hard-disk digital audio players, which include the Apple iPod, Archos Jukebox 5000, 6000, Studio, Recorder, FM Recorder, Recorder V2 and Ondio MP3 players. While it adds features in some cases the RockBox firmware improves on the player abilities.

On the Apple iPod for example, RockBox adds gapless playback, OGG/Vorbis, FLAC, AC3 file format decoding, a 5-band parametric equalizer, replay gain as features. On the Archos Rockbox improves playlist loading speed, provides automatic volume control and even adds a sleep timer.  

It's completely free for download and use, but one word of caution though, RockBox is highly experimental and is not an add-on, so it replaces your existing firmware - the hardware operating system on all these MP3 players.  It could void your warranty/damage your player and what you do is at your risk. If you still feel convinced, jump to RockBox