Fine Art America
To date, I've personally only tried Fine Art America (a fellow artist was familiar with it) and because they'll produce and ship affordable prints of your work directly to customers. It's also free to post your work and if you want to be part of the print service, I think I paid about a $30 annual fee.
At first, I was happy with FAA. My traffic (views of my art) was very high (it still is actually). Several of my pieces (such a my lucky rabbits above) have been viewed by well over 1,000 visitors, but those have yet to translate into sales.
However, shortly after I joined the site, I started to have trouble with FAA. Often it's slow, or it won't load at all (and yes I do have a very high speed cable connection and have cleared my cache :)). So, I can't help but wonder if other visitors and artists have similar troubles with the site.
Because of that, about a week ago I did a quick search for other potential online galleries. I've not had a chance to look at any of these sites carefully yet, so am not endorsing them here, but here's a list of places to sell artwork online I've found so far:
If you have a positive experience with any of these, please feel free to send your comments. I'd love to hear of a website that's working well for you and your art.