Kazaa is king. Better than Napster, except for the spyware. Install it and then get Adaware and then get Kazaa Lite.
The regular Internet is terrible for MP3 sharing. I know from the horrible experience. Sometimes files are not what they say, quality is not reliable, and selections are very limited. And the popups are very annoying. Kazaa or WinMX are best because they have a massive user count. Right now on Kazaa, there are 2,668,001 users on and they are sharing 422,465,000 files taking up 2,913,472.0 GB. Good enough?