http://www.buzzfeed.com/kaylayandoli/robin-williams-best-cameo-in-friends-history#3csj7h0

The late actor crashed Central Perk in Season 3, and it’s the best.

I'm glad I saw that. Even though I never cared for Friends that was funny.

http://whatculture.com/tv/10-robin-williams-tv-appearances-must-watch.php

A collection of Robin's guest starring roles and various television appearances.

Here are Robin and Billy Cystal.