If you don't want something everyone else can grab for free, you either have to make it yourself, or pay for it. Getting exclusive rights to a professionally produced photo can be expensive, so there is a compromise - royalty free photos. You have to pay for the rights to these, but you can use them confident that they are less likely to be seen elsewhere - and if they are used elsewhere, it's because someone else has paid for them too.
This type of resources all tend to have extensive search facilities and boast a wide range of subject matter, so to some extent are all very similar. Which one you find best is a matter of personal taste as much as anything.
AcclaimImages A large choice for the professional.
Corbis A big name in photography. This really is the quality end of the market.
Fotolia Simple pricing scheme, and an excellent search and preview facility.
Fotosearch Individual images or bundles, royalty free or rights managed, there's plenty to choose from here.
Getty Probably the biggest name in stock photography.
Photos.com If you really want value, their subscription model lets you pay on a period subscription basis, rather than paying per image.
StockXpert Price dependant on size - if it's for web only, very good value.
Veer A resonable selection, also includes some interesting illustrations.