I use Blubster. Its the first program I use to find stuff as it has the best quality songs and pretty good download speeds. Its rare to get a song from there that was not recorded properly unlike all other p2p programs I've used before. The only drawback is that its not as popular as some other programs so you can't always find what you want.
Other than that I use Kazaalite and IRC for everything else. Of course the biggest factor for me all comes down to what type of music you are looking for.