Shutterstock is the most popular microstock site, the iStock is a close second. iStock (or simply IS) is my #2 seller overall (7 months), but has been tops over the past 22 weeks. Plus, they have had a pretty steady upward trend. These two make up better than 90% of my total microstock earnings. Sales at IS aren't as broad as SS, but my dollars/sale is almost triple. The uploads aren't quite as easy, and their keywording is a drag. BUT, you just can't argue with the performance!!
My top sellers at IS: