[ there's mainly two different ways of artists getting mainstream exposure: word-of-mouth, and payola; forgive and correct me if I'm wrong on that]
Besides college radio and message boards [that have been mentioned]
The suggestions here are very good ones, here's some of mine that I use as well:
The radio show,
sounds eclectic, is a great source for up & coming artists that have risen in popularity mainly due to word-of-mouth. It's another reason you should donate and listen to NPR !!! Last time I was listening to it, they featured the arcade fire, which is definitely one band that will be on the mainstream radio/mtv2/fuse circuit within a few months.
Another source is pitchfork media, which has album reviews/ music news/info on a diverse selection of artists. A few of them, I witnessed, like kanye west and bright eyes, have risen up to the top.
My other source, which is much more difficult to find, are "va-promo_only_modern_rock_january-2005" CD's. These, pretty much, are a cd of about 15-20 songs, released every month, in the categories of modern rock, urban radio, and top_40. A few weeks later, you'll notice these songs being played like hell, 2 or 3 times a day, at least, on your local radio stations [the ones that owned by radio1/clear channel].
I presume they're given out to the stations and pretty much told [in some way or another] to play them.
They're [the cds] pretty hard to find, imo, usually the only places you can find these at are on newsgroups and IRC channels. If you want more info on these, I might be able to help, just pm me.
Though I don't use this source, I've heard a lot of artists, like phantom planet and rachel yamaguata [sp?], to grow in popularity after being featured on the tv show 'the o.c.' [popular among the mtv/vh1-watching demographic from 16-25]
Plus, another great web radio site is woxy.com - they play some music that's been released or has more of a following in the UK and not yet in the us; as well as indie and eclectic stuff. I recommend woxy enthusiastically.
Lastly, there's the 'New Music Monthly' by CMJ - College Music Journal, a monthly magazine featuring album reviews and info about up and coming artists - each issue also comes with a cd with a few songs on it. I'm not sure how much subscriptions cost, but my local public library subscribes to it, so I just get it from there, albeit a month or 2 behind.
After reading this thread, how about making another one for us to guess who will be popular in the next say, 6 months ?