Here is my list in order of download
Dreamstime
123RF
Big Stock Photo
IStockPhoto
SnapVillage
MostPhotos
But in when in comes to money then
MostPhoto is probably on top of the list. They just pay a lot more per download.
Two news sites that I think may do well in the future (at least I hope so) are:
Yay Micro
Cutcaster
You can find larger list here
H.Lehnerer Photography - Resources