Hey, It's That Guy!: JG Quintel worked on the show as a creative director, which explains a lot about why Regular Show is so weird (well, that, and the two student films Quintel did: The Naive Man from Lolliland and 2 in the AM/PM). His influence can be seen in the character designs.
Gregg Turkington frequently contributes miscellaneous voices to the show. You may know him better as everyone's favorite anti-comedian, Neil Hamburger.
The legendary June Foray provided the voice of Ruth in "Bubbie's Tummyache". The fact that the episode aired right after a Looney Tunes episode that had Witch Hazel made it all the more easier to pick up. Considering she's 93, her line as Ruth, "I just want to have a little fun... before I die", is a one of the darkest and saddest casting gags ever.
Screwed by the Network: For one thing the shows timeslot changed several times over the course of its run, it rarely IF EVER got any reruns and as mentioned on the main page, unless you subscribed to Thurop's deviantART account, you wouldn't've known when the Series Finale aired!