Clicking links certifies you're 18+, want to see really nasty sexually explicit material, won't let minors see it, and it's legal for you to view where you live.

Mr. Skin is the authority on celeb nudity. He's a regular on Howard Stern and many other morning radio shows. This is his definitive collection of over 10,000 pics and 6,000 movie clips featuring nudity or partial nudity. Forget fast forwarding through movies, he's already clipped just the good parts! 7 day trial for 99 cents a day. (I can't tell you what 7 times 99 cents would be, because it would seem to expensive then...but cheaper sites are below).
Roger is kind of a cheaper knock-off of Mr. Skin. Still good stuff, but not as careful about filtering out the fakes from the real pics, or an actress' body double from the real McCoy, so you're liable to see Britney's head on someone else's body and such. (Hint: if you see her in a video her giving alternating blowjobs between two guys while riding a third, it's not really her. Well, yet anyway). $2.95 for 3-day access.
Celeb Ticket is another pro site with an emphasis on younger stars. They have only genuine photos, no "photoshopped" fakes, along with some good recent TV clips. (Award ceremonies are great for catching breasts popping out of dresses!) $2.95 for a 3-day trial.
True Celebs is run by one of the big net porn kings, so in addition to a nicely organized list of a ton of celebs, it also has an enormous hardcore archive of non-celebs. $2.95 3-day trial.

True Male Celebs is True Celeb's male companion site. Get the latest on Ricky Martin, the backstreet boys hot tub romp, Brad Pitt sunning au naturel, David Duchovny relaxing is his birthday suit, and a whole lot more!