the only one i know about is limewire.... but be careful of that internet providers are starting to track the usage and can suspend your acct for using it (i guess its illegal)
stay away from anything that makes you download other things sometimes they use spam to pay for thier operation (this pertains to the free ones only)
Kazaa used to be a big one for it
i used to use WINMX but they got shut down
right now i am all about itunes (i have an ipod) but its .99 a song
you can get gift cards for itunes tho
same w/napster
and alot of companies will give you 50 or 100 free songs when u first sign up
and dont forget u can upload ur personal cd collection too
hope this helps!