Look up the touring schedule for bands that you like, and see who they play shows with. Look through their discography and see if they have any split EPs or collaborations with other bands. If you find yourself enjoying a lot of bands on a particular music label, check out the others, small labels are usually based on groups of friends that enjoy each others' music.

I also use lastfm, bandcamp, and various subreddits.

turntable.fm and mumuplayer.com are great ways to find new music if you can get a few people to join you. It's like going to a party and rotating stereo control, except you don't have to all be in one place, and the software manages everything so you don't have to plan things and constantly plug/unplug devices.

Some friends of mine are a fan of generating a Pandora station with one song suggestion from everyone in the room, it usually generates an eclectic mix but it's all fairly major artists.

I torrent most of my music on private trackers, so if I find myself downloading a lot from a particular user, I like to go through that user's upload page and snatch things that look interesting.