I use Shutterstock. They have a reasonable membership plan ($224 a month for me), where you can download 25 photos / illustrations a day.
They aren't as good as iStockPhoto, but I am way too cheap to use iStockPhoto regularly. Plus I found their credit system infuriating. If you find something you like on iStockPhoto that isn't labeled exclusive, search for it on Shutterstock. Sometimes it's there for a lot less money.