What are the best online poker sites? How are the top sites different?

First, check out our mini-reviews of the top sites in the bonus code center.

A great resource for comparing the popularity of the different sites is Poker Site Scout. It is like TV Nielsen Ratings for poker sites. Here is a recent Site Scout snapshot of real money players at the top sites (not including tournaments):

Also, the rec.gambling.poker FAQ has links to other sites that have reviews of online cardrooms. A good example is The Poker Project.