ClipArt.com has two advantages over other clip art websites. The first is it's huge variety - when they say over 10 million images, they mean it. You can go to the search page to find them (the high-quality search is another plus). The second major benefit is its subscription model - for the price of one or two images elsewhere, you can get access to all 10 million images and download what you need. All these things combine to make this a quality clip art web site.
If you need something more professional than clipart.com, photos.com is your ticket. Again based on subscription, here's access to professional quality photos. Highly recommended.
If you don't like the subscription model (if you need only a specific image, for example), check out Stockxpert.com.