I compiled a list of my top stock photography sites. They all vary in different features. I recommend checking each one out, however I suggest maximizing your profits by joins more than one if possible. Good Luck!
ShutterStock is one of the bigger microstock photography sites and has a high sell rate. They do have a very rigid photo review process however, but once accepted you're photos will be well marketed to a large number of buyer.
iStockphoto is another one of the big sites which has its upside and downside. Thousands of potential buyers browse big sites like this, however they tend to be more picky than the smaller sites. The registration process may take slightly longer than other sites, but once you're in and get some photos accepted you'll see the money start rolling in quickly.
Fotolia is a relatively new site that is causing quite a stir in the stock photography community. They are well organized and have a quick registration process. Although they have a moderately rigid photo review process, they do not penalize you for submitting photos that don't get approved. Fotolia is growing and has a good customer base which translates to a high number of sales.
FeaturePics has the least rigid photo review process and is your best bet for getting photos accepted. The also have a fairly high commission rate. FeaturePics frequently tried new ways to sell your photos and offer you the option to participate in new projects they have. I recommend taking advantage of their new ideas, it will just boost your revenue.
123rf is right in the middle when it come to their image review process. They accept more than most other sites on this list, but do reject a good amount as well. I'm not too familiar with their sales volume as I just joined this site, but it won't hurt to give it a try.