Fotolia

This is one of the first micro stock sites I started contributing to. It's also one of the ones I buy from as a designer. Their qualifications for submitting files is pretty good. They also provide good feedback as to why your images are declined, which is very helpful when starting out. They have good photo management tools and options as well as decent traffic, this is one of the more profitable micro stock sites.

Dreamstime

I've had an excellent amount of downloads from Dreamstime.  Their approval process is also pretty good although the backend might not be as easy to navigate.  They also pay out pretty good.  I sell a few less photos, but each photo sells for more with Dreamstime.

123rf (formerly 123 Royalty Free)

123rf is a great site for buying photos, I actually use them as a designer.  The reason is they have great subscription plans that let me buy photos without worrying how much they cost or what size to buy them at.  While you don't get paid as much for a subscription download, people with subscriptions tend to download more photos.  I used to just download photos randomly at the end of the day knowing those credits wouldn't be used anyways.  Their management tools aren't as intuitive or easy to use, but they approve photos relatively quickly and it's a good site to start with.

Shutterstock

Shutterstock has great earning potential.  Their submission standards might be a little higher than the above sites, so if you're having trouble getting your images into the above sites you might want to work on your micro stock photography a bit first.  Once in with submitted images they have a great user base and have high earning potential.

Crestock

Crestock has been around for a while.  Some people have done well with them, others haven't.  They seem to be able to sell a few of my photos so I think it's worth the upload.  Getting your photos on a number of quality sites is the key to success.  All the sites I've listed are very reputable.  The last thing you need is a site that goes out of business after a year, doesn't pay you, or doesn't count your downloads properly.