I've discovered tons of new music through last.fm. Not by using their "recommendations", but by finding people with similar tastes and see what they are listening to.
If you have a specific type of music you know you like google can find a ton of stuff too. I was looking for some 60s/70s psychedelic rock, so I typed "best psychedelic rock" and found hundreds of albums to check out. There are a lot of forums out there too, although they tend to be restricted to a single genre each. I've found a lot of music that way as well.
I'm finding more music I like now than I ever did when I was just relying on friends and opener bands at shows. Then again, I like a huge range of music (psych rock, classic rock, some punk, some rap, reggae, psytrance, metal, blues, jazz, folk, classical indian, psychill, ambient, classical), which does make things easier. Even liking all that music, there isn't much I like on the ClearChannel radio though.
It can admittedly be time consuming though, but that's only natural. Not all music you find you are going to like. It just depends on how passionate you are in regards to music.