Rockbox started as a reverse engineered alternative firmware for certain Archos MP3 player/recorders -- now Rockbox varieties support machines from Archos, iRiver, iAudio, and Apple. What Rockbox provides for iPod users is an alternative operating system for several models of iPod. It works with the two most recent generations of (full-sized) iPod, as well as with the nano and Mini varieties. James Kim of CNET News has posted his Rockbox impressions:

In its current state, the Rockbox interface is utilitarian, graphically sparse, and reminiscent of a command-line system, and it lacks the flash of modern commercial MP3 players. But where it comes up short in pizzazz, it more than compensates in options and settings, many of which would make the default iPod jealous. Using RockPod is fun, and though it doesn't benefit from a seamless iTunes relationship (you can, however, manually transfer songs within iTunes) and is missing a few familiar features that make the iPod so popular with the masses, I personally dig it--enough to use it exclusively for a while.

Some of the Rockbox's key features are Karma-like: OGG and FLAC support, as well as gapless playback (dance mixes on a standard iPod are anything but seamless). You also get crossfading options, fade in/out, a user-definable antiskip buffer, and much more--all on a spartan interface that's simply not as pretty as the iPod's. But the Rockbox is fairly easy to use, considering all the extra features you get.

You get photo playback--no thumbnail grid views--but you can zoom in and tweak slide-show settings. File/folder organization is completely customizable, and you can browse by ID3 tag data. Rockbox even offers a choice of playback screen skins (again user-customizable and created); some include dancing graphics (a.k.a. DancePuff Duo), while others show a battery meter with the actual time remaining. The Rockbox GUI is exceptionally customizable as you can see in these beautiful 5G iPod example themes. Incidentally, the iPod's battery life takes a serious hit when using Rockbox, but this is a known issue that will be remedied by the time Rockbox for iPod officially launches later this year. There's playlist creation, bookmarking, and a library of plug-ins ranging from games and a calculator to a very cool oscilloscope. But most of all, it offers a hard-core control over audio playback.