The Tappan Collective is a site curated by a former fine art student who wanted great, accessible artwork. She has paintings and drawings but I especially love her photography collection.
Bowerbird is another favorite of mine. They have a team of artists who whip up fun, whimsical paintings at super-reasonable prices. I love their peony series. They also have some gilded monarch wings-- very flutter by!
Society6 is the Etsy of the art world. There's so much sifting involved that I normally throw in the towel two pages deep into hunting. Half of it is hideous. Lots of odd, Urban Outfitters-eque things, like mutated doll drawings, pandas wearing backpacks, and stuff like that. Boo. However, they do have some funky, out of the box pieces as well. Plus, you can get them on a canvas or have them printed on cell phone cases, iPad covers, or even pillows. One of my favorites there is Ben Giles. He takes vintage photos and does cool collage work.
I'm sure most of you are familiar with this site. One Kings Lane is flash sale site so you have to pull the purchase trigger asap if you want to get your hands on something. They're currently having a sale titled "Boho Bedroom." Based on the name, art isn't the first thing to come to mind but they had some bright pieces to liven up a neutral boudoir. See anything you like? Jump on it! The sale ends on 3/31.
The Animal Print Shop has a huge following among moms. They have tons of great images of baby animals begging to be hung in a nursery. But I think they're grown-up worthy too. I'm especially loving all of their tropical birds.
Caitlin McGauley is an illustrator who does feminine watercolors in summery, juicy fruit hues: think peach, watermelon, honeydew, and cantaloupe. She just started printing her designs on little trays. I can't decide which I like more! I think these would be great in an office.
20x200 is another favorite site of mine. Tons of modern photography. It's currently under construction so I couldn't show you my favs. But definitely take a look!