As a web designer I use Istockphoto and Bigstockphoto the most – think Bigstock don't require exclusivity.. Istock now charge more per photo if they're exclusive, but seem to have some non-exclusive ones as well.
Best ones to sell are boring generic ones.. beautiful receptionist next to a phone and computer, a selection of ethnically diverse office staff around a computer etc etc..
Your best bet IMO is to think of who your target audience is.. most buyers, judging by volume sold, seem to be web / graphic designers looking for generic photos for the "About us", "Our Services" and "Contact us" pages of their client's brochures/websites.