Nicotine is a client for the SoulSeek peer to peer file sharing network. Nicotine is based on pyslsk, the original python SoulSeek client. Pyslsk is now no longer updated, and nicotine has picked up where pyslsk left off. Nicotine, unlike pyslsk, uses GTK2, and as such is a much more attractive program to use. It has some issues with speed and memory usage, especially when scanning shared files, and returning search results. Nicotine can also run on OSX, using Fink. The nicotine chat room is a somewhat friendly place to go for advice on setting up the program on OSX, assuming you can get to it in the first place.
Nicotine is currently at 1.0.8rc1, and avaliable at http://nicotine.thegraveyard.org. It is GPL'd and supports psycho extentions to improve its speed. Nicotine is eventually planned to be replaced by museekd, a SoulSeek daemon programmed in C++. Museekd will be controlled through front-end applications. Museekd, however is not available as of yet.