I recently joined a music club with a bunch of friends where you create a playlist that you share with friends. People go check out the songs and it's a way to discover new music. (8Track allows you to do this and iTunes will also let you upload playlists.) We're lucky that we have Delicious keeping our little group of people on task to make sure we pull together playlists each month. I'm even luckier because I've found two great resources for free music.
A while back Rage Against the Minivan featured a link to Downplayer and it's been wonderful! I check it out Monday thru Friday for 10 free tracks. And, before you go jumping to conclusions, yes, it is totally legal. Downplayer just links out to other sites featuring free music. Often the sites they link to are the music artists' sites.
Another site I've recently visiting is Radio Amp 97.1. They feature one free song to download each day.
Between these two sites I've expanded my music collection a ton but also gotten introduced to some great new artists and fabulous songs. Case in point: King of Diamonds by Motopony.