clipped from: www.rockbox.org 
Rockbox for the iPod 4G, Color, Nano and Video models all have a wide range of codec support - MP3, Ogg Vorbis, Musepack, FLAC, AAC, ALAC, AC3 and WavPack are all supported in Rockbox. Additional music playback features include gapless and crossfading, as well as a five-band parametric equalizer to fine-tune sound output. iPods also get ReplayGain support for volume level normalization, and a large number of plugins, including games, applications, and "demos" - have some fun when you've got some free time to kill, open files with the text viewer, make use of the stopwatch, view some JPEG photos, or turn your iPod into a desktop clock. On-The-Go playlist creation and adjustment, full file bookmarking support and much more.
| Feature | Rockbox | Apple iPod stock firmware |
|---|---|---|
| Gapless playback | Yes | No |
| MP3 Decoding | Yes | Yes |
| MP3 Realtime Encoding | No | Yes |
| OGG/Vorbis Decoding | Yes | No |
| FLAC Decoding | Yes | No |
| WavPack Decoding | Yes | No |
| WavPack Encoding | Yes | No |
| AC3(A/52) Decoding | Yes | No |
| AAC/MP4 Decoding | Yes (13) | Yes |
| ALAC (Apple Lossless) Decoding | Yes | Yes |
| WMA Decoding | No | No |
| 5 Band Parametric Equalizer | Yes | No |
| On The Fly Playlists | Yes | Yes |
| Crossfade & Crossfeed | Yes | No |
| Voice UI | Yes | No |
| ReplayGain | Yes | Yes (12) |
| Video viewing | No | Yes (11) |
| Picture viewing | Yes | Yes |
| Text viewing | Yes | Yes |