Benjamin Meyer, author of Kinkatta (the app formerly known as Kaim, renamed at the kind suggestion of AOL), is ready to present his next project, Sondra.
Sondra is an on-the-fly MP3 playlist generator that is really quite original in its concept. You can rank a song while listening to it, and this personal ranking, along with some other pertinent information such as the number of times you have listened to the song, is written to the MP3 using the new ID3 format. Sondra uses this information to generate the playlist on the fly, creating the amazing feeling that one is listening to a radio station but one that is particularly geared towards one's preferences. Sondra has a KDE-interface, a command-line interface, and the backend is implemented as a library, so anyone can use it with maximum flexibility.
According to Benjamin, this concept may well be unique and original enough that it probably hasn't been implemented elsewhere (including Windows-land). Currently, Sondra is tied to XMMS, but should be easy to generalize to use your favorite player. I also wonder if Sondra could be made to use Ogg Vorbis, given the patent troubles with MP3.