There's the three big ones: http://www.Shutterstock.com, http://www.iStockphoto.com and http://www.Dreamstime.com
For ShutterStock and iStock you have to get accepted before you can start to submit real pictures and sell them. Dreamstime there is no such thing and you can just upload pictures that should be good enough as stock and they'll put it up.
ShutterStock requires you to send in 10 pictures of different things to get approved. You need 7 out of 10 images approved. For me, it was easy to get in, I even added 'filler' pictures because I had 7 good ones, and 3 bad ones. Nine of them got accepted. I get an average of around 5 downloads a day, this is my most selling stock site and it's easy to get in to.
iStock is really hard; they ask for 3 pictures. I've submitted twice now, and each time they've sent me back a message saying my images are too 'similar' (how a valentine's card and a spa leaf are similar is beyond me). I wouldn't try this one, but this is the second biggest stock site out there.
DreamsTime doesn't have an acceptance program; so you can just start uploading pictures and wait for them to be accepted or rejected. However, I've uploaded several pictures now for a week, and still have no sales. I think it's a rather slow site, but I guess for you it's good.
Try both ShutterStock and Dreamstime at the same time. If you think you're good enough, try iStock.