Here are some links to the most popular microstock photography sites, along with with comments about each site. If you are new to microstock photography, then you do need to sign up to the best sites, for there are dozens of them and only a handful that seem to perform well for photographers. Some sites generate almost nothing in the way of monthly sales.

Shutterstock
Shutterstock is the most lucrative microstock photography site online, and earns practically every microstock photographer I know significantly more than any other microstock photography website. Sign up now, then read their forums before submitting any images to ensure 7/10 of your images are accepted for if they are not, you face a 30 day account suspension before being able to try again. The key is understanding what makes a photo technically good. Shutterstock lets commercial buyers purchase a monthly subscription, which entitles them to download up to 25 images per day, which boosts sales in a big way! Although they only pay $.25c per image, with a portfolio of a few hundred good images you could be earning a few hundred dollars there per month.

Fotolia
Nice interface, good sales, good review times, good affilliate commissions - this site is usually within everyones top 3 best sites regarding photo sales.

Dreamstime
Somewhat slower than shutterstock, but a good earner, depending on the content in your portfolio. I know many people who earn almost as much from dreamstime as they do from Shutterstock. It is one of the better sites, so is definitely worth signing up to.

BigstockPhoto
I quite like Bigstock. It is very easy to get your images approved there, although they sometimes slow in approving images. Possibly the best site to sign up to first if you are a beginner as they are not as fussy as shutterstock or dreamstime, although sales are fairly slow. The site is easy to use, and it is for this reason that I like it.

IStockPhoto
Another good stock web site, worth signing up to. Although the number of photos you are permitted to submit per week is rather small.