For those of you who don’t care for the stock interface or firmware on your digital music player, there’s the Rockbox firmware, which works on many Archos, iRiver, iPod, iAudio, Gigabeat, and Sansa devices. The firmware features extended codec support (OGG, Flac, etc.) plus enhanced audio magic, MPEG video support, tagging, plug-ins, playlist creation, games, and more (see the full feature list here).
Plus, it’s open source so it gets developed on a fairly regular basis. Version 3.0 was recently released, adding new software decoding features, a new installer, more codecs, and various miscellaneous updates and improvements.