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.

Roger is the man on celeb nudity. The definitive collection of pics and movie clips (just the good parts!) And if you want the lowdown on Britney's latest nipple slips, Roger's got the goods! $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.
Illegal.net is more into sensationalism, with many real pics but also many of dubious authenticity (digitally retouched fakes, and look-alike videos). On the other hand, it's still fun to watch a bubblegum pop queen look-alike take it in every hole! $4.99 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!