Hey, It's That Voice!: There have been a few "celebrity guest appearances" in some of Psy's fandubs and Kirbopher's flash movies. Mike Pollock (voice of Dr. Eggman in Sonic the Hedgehog) and Kevin M. Connolly (Kain Fury in Fullmetal Alchemist) have provided voices for Go Go Parody Rangers!, and Egoraptor (of The Awesome Series fame) voices Zordon in that series, as well as providing the voice of Snake (Metal Gear Awesome style) in $00pah NiN10Doh!. Kyle Hebert (voice of Teen Gohan in Dragon Ball Z, Kamina in Tengen Toppa Gurren Lagann and Kiba in Naruto) voices Tabuu in $00pah NiN10Doh! and Optimus Prime in Transformers: Bastardized!, Psy's fandub of the animated Transformers movie. Doug Walker also provided his voice for a short clip in the Parody Rangers movie, for an animated version of his character, The Nostalgia Critic.
Kirbopher himself may qualify if you have seen the Pokémon anime.