Actually Andy Dick in ST:VOY.

And I hate Andy Dick.

speaking of Andy Dick and Star Trek.....
Brent Spiner in 'Dude, Where's my car?'

Neil Diamond in saving Silermen

Although it's not really a cameo, I liked the part in Twins when Arnold is checking out the big city while looking for his Twin, and he comes across a movie poster for Rambo, with Sly Stallone looking damn cut up. Arnold looks at the poster, then looks at his own huge bicep, and disses Stallone. Hilarious!

Jay & Silent Bob as themselves in Scream 3