Rockbox
“Rockbox is an open source replacement firmware for mp3 players.”
Available for: various Archos models, iRiver H100 and H300 series, iPod 4G, 5G, Nano, iAudio, and in the future perhaps even more.
Rockbox is much better than the original firmware of my iRiver H340; it boots up the player at an incredible speed, has many more options for playlist generation and makes this ugly, yet lovable gadget on the whole much more customizable (alternate fonts for the display, equalizer, fading, bookmarks etc.); a database that makes music searchable by artist or song title is also in the making. It also supports AAC/MP4 files, which the original firmware didn’t. Oh, and there are games, if needed.
[File under: how open source can change, er, improve, your life, grup.]