- Omar is in at least two eps of Law & Order and both times he's a jailhouse visit LOL. - The guy who plays Clay Davis is in at least two eps. In one of them, he's the killer and Bunk is his lawyer. (Bunk shows up in a few other episodes too) - Idris Elba is in the one with Kerry Washington. - Daniels is in a couple of them. Once as an African soldier. - Pearlman has a quick cameo as a beatcop in an early season. - Bubbles popped up in one.
And that's not including the spinoffs. I'm sure they pop up on those as well, I just don't watch them.
But the same happened with The Sopranos. Pretty much half of the working actors on the East Coast have appeared on a Law & Order somehow. Basically, if you're black or Italian, odds are you'll pop up on a crime show.