Jonathan C. Osbourne - Voice of Gaimon & Shojo in the FUNimation dub of One Piece, played Tron villain "Sark" in the tail end of scene 2. Funnily enough, my Dad (who previously played Lord Zedd on the phone in GGPR2) was the voice of the Master Control Program.
Martin Billany - Better known as LittleKuriboh, creator of "Yu-Gi-Oh!: The Abridged Series", going along with his joke of Dan Green being one of the minions of Ivan Ooze (which he wasn't, but anyway...) and thus I designed one of the Rats with Yugi's crazy hair.
Joey Blanchette - Also known as LegendaryFrog, one of Newgrounds' old school animators from things like "One Ring To Rule Them All" lent his voice to the talking Gnarwal sword. He ended up using a voice like Homsar from Homestarrunner, hence why it's drawn that way.