The latest issue of Radar magazine -- yes, it's still being published! -- has a long article on them (available online). Including the most infamous of them all:

Anyone who has ever performed stand-up is familiar with the red light, the universal signal that warns dawdlers it's time to wrap things up. In the '80s, comics at the Hollywood Improv came up with a novel use for the light. When shining steadily, it had the conventional meaning. But if the bulb began sputtering, it was the comedic equivalent of an air-raid siren, warning performers to lock up their original material immediately unless they wanted to lose it to a master thief.

Robin Williams, comedy's most notorious joke rustler, was in the house.

Radar paints him as less a thief than some sort of semi-autistic who can't keep himself from absorbing all the jokes he hears and reflexively spitting them up onstage. (Kind of like me with liquor.) Also mentioned are Mencia, Dane Cook, Denis Leary, and others. It's a tough thing to prove, and even if you do, in the comedy world there's not much you can do about it even if you want to. As Louis CK said about some Dane Cook bits that were suspiciously close to his own:

"Okay, this kid is stealing from me. And making lots of money. Three bits on one CD," he wrote on A Special Thing's bulletin board in 2005, adding, "Just so you know, guys, I'm not going to do anything about this.... I'm not going to court over a bit called 'Itchy Asshole.'"

P.S. Joe Rogan gives his side of the story. Here's the thesis:

For someone who is not a comic, and doesn't understand what the big deal is about comics stealing jokes, please let me try to explain it to you.

It's hard to come up with material. It takes a lot of work, a lot of rewriting, sometimes an approach bombs and you have to rethink it, and often it's a long process until you get to a finished product...

Now, when you work hard on a bit and polish and craft it, and then someone just disrespects the whole process, steps in, steals it and performs it as his own, that's a pretty intense creative violation.

I've been in the back of a club while a guy was onstage doing one of my bits and believe me that's a shitty fucking feeling. It's like watching your girlfriend getting fucked onstage by a liar while the whole audience cheers.

He also says Mencia called him out, whereas I assumed Mencia was doing a set and Rogan just jumped onstage and got in his face.