First is Clickforart, a nifty site that I discovered tonight. Clickforart is bringing good art to the masses. you can choose art by the artist, theme, style and size. You can then get it framed and sent to you. Their tagline, "real art for real people at real prices" has a nice ring to it.
And who can forget the awesomeness of the Thumbstack Press, the down-to-earth online store where you can buy some great AND cheap prints (plus a few originals).
Thumbstack seem to have an ever growing stack of cool artists.
Finally we have the AMF Gallery. Perhaps not the prettiest site of the group. But with artists like Daniel Lim on board they are doing OK. The art fits within everyones' favourite new art genres, Lowbrow, Pop Surrealism and Street Art. AMF has original artwork - not just prints - unlike Thumbstack or Clickforart.
BTW if you are an artist who is thinking about setting up your portfolio online, and would like a few suggestions, the excellent Creep Machine blog has a terrific post about the best ways to show your art on the web.
I'm such a nerd - I love the internet!