Here are some more that I've learned how to do since my last post:
-William Shatner
-Skeletor from He-Man
-E.T. the Extra Terrestrial (PHOOONEE HOOMMME!)
-Bill Clinton
-Lindsay Lohan (The Robot Chicken version only)
-Superintendent Chalmers (The guy from The Simpsons who yells "SKINNER!")
-Zoidberg from Futurama
-Lammy from Happy Tree Friends (Basically a sheep sound in a higher pitch tone)
-The battle droids from Star Wars
-Snarf from Thundercats (This one is so fun to do, snarf snarf)
-Dr. Claw from Inspector Gadget (His voice actor referred to the voice in an interview as "an evil Barry White impression" lol)
-Severus Snape from Harry Potter (People say he doesn't have a catchphrase, but he certainly says "Potter you fool!" a lot)
-Poof from The Fairly Oddparents (This is a FAIRLY easy one. Excuse the pun lol)
-Ted from the movie Ted (Pretty much the same as Peter Griffin)
-Solid Snake from the Metal Gear video game series
-The Robot Chicken nerd
-Groot from Guardians of the Galaxy (To get this impression right though, you can only say two things in this voice:I am Groot and every great once in a while We are Groot)
-Alfred, the butler from Batman (Specifically the Gotham version)
-Howard's Mom from the older episodes of the Big Bang Theory
I've also found out that Cub from Happy Tree Friends sounds like a mix between Giggles, Elmo, and Pikachu when I do the impression. Yeah, not one of my best