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 TheAwesomeSeries fame) voices Zordon in that series, as well as providing the voice of Snake (MetalGearAwesome 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.
Internet Counterattack: Ever since this post was leaked onto Tumblr, it led to dozens of people sharing stories regarding rather horrible experiences at the hands of Bryon, ranging from abusive relationships (both sexual and non-sexual), pedophilia, manipulation, murder threats etc. This has only become worse through a case of The Streisand Effect when it was later revealed that Bryon had threatened legal action towards some of the people who shared their stories about him and tried to hide as much of it as possible through copyright claims against Youtube, leading to hundreds of people reblogging and sharing the post to as many websites as possible. It has given him and Fireball 20 XL a bad reputation in many circles to say the least.
SuperPsyGuy has since shut down all of his domains (including the Brentalfloss comic, GG-Guys and fireball20xl)