scads of 'em. Depends on what you want to do.
if you only have a little money, go to one of the chain book stores. There's several magazines that allow you to do computer music. Sampled instruments, beats, drums, etc. Mixers, compressors, effects, etc., are usually offered on a CD.
Failing that, try going to Nonags, SourceForge, etc., for plenty of programs that will do the same things. Also, search on 'open source' and 'music', and you should get plenty of hits.