Founded by two L.A.-based twentysomething art enthusiasts, The Tappan Collective sells limited-edition prints and original work from young, emerging artists. A lot of the art (which starts at $20) has a cool Los Angeles vibe — muted pastels, retro-looking beachscapes, and so on — perfect for making your walls Instagram/Pinterest-friendly.
Little Collector is aimed at kids, but shouldn't adults be also allowed to own "Girl With A Pearl Earring" recreated in balloon form, or this piece made out of Barbie toothpaste? The selection of works for sale is playful and kid-friendly, but not childish.
If you like words on your walls, you will like Hu2. The Paris-based (so it has to be cool) site sells wall stickers, typography posters, and other (mostly) text-focused things to decorate your home.
Support the artists! Saatchi Online, a outshoot of London's famous Saatchi Gallery (which its founder donated to the public in 2010!), sells work directly from artists, most of whom aren't represented by galleries, and lets them keep 70%. For most of the pieces, both the original work and prints are available.
Artspace's staff curators work with museums and galleries to handpick the thousands of works featured on the site, so you won't be combing through junk. The quality is high, which means prices can be too, especially for established artists. But! The curators also make a point of featuring newer, emerging artists, so if you're looking to invest in art that could end up becoming more valuable with time, this is a good place to start.
If you want to buy a Warhol (or a Damien Hirst, or a Cindy Sherman, or what have you) from the comfort of your couch, Artsy has you covered. If not, (and there are some lower-priced items to be found) it's still a great place to browse, see what's out there, and learn about the art market. Functioning kind of like a huge online international art fair, it's like a trip to the MoMA with all the comfort of scrolling through Pinterest.