Well, there are a bunch of sites with art outh there, but of course the large the site, the harder it is to find things you like.
http://www.deviantart.com - is a general art site, with millions of members with galleries. There is quite a lot of good content there, but finding it is the hard part. Their search engine sucks even when it works, which it doesn't right now. I guess you can look at the Daily Deviations, and maybe you'll see stuff you like, and work your way backwards.
http://elfwood.lysator.liu.se - If you like fantasy art, Elfwood usually has a pretty good selection of amateur fantasy art. Since they're rather anal about their approval process, most of the real crap is kept out, but there's always plenty of average stuff. At least they have a working search engine though.
http://www.fanartcentral.net - I only joined a few days ago, and from what I've seen so far, the average work is somewhat lackluster.
Naturally, I've just posted galleries at which I am also a member, but I'm sure that won't make your viewing experience any the lesser.
Edit: Of course, if you're interested, here are mine:
Elfwood
deviantART
Fanart Central