I just came across a pretty cool new site called Cyclops – http://cyclo.ps. It lets you search through six different stock photography sites, all with only one search!
Included in the search so far (and you can select/deselect any of them for each query) are Photos.com, Shutterstock, iStockphoto, Fotolia and BigStockPhoto. The website combines the search results into one cohesive list of images for you to go through. Very convenient and they claim to be working on adding more sites. They’ve also got a blog if you want to see what they’re up to. Check ‘em out!