Daran Norris does plenty of voicework (J Jonah Jameson in Spectacular Spiderman, Imaginary Man in Foster's, and Dick Daring in the Replacements are some non-surprising ones if familiar with his voice). Some of his more surprising stuff include voicing many named characters in Time Squad (who knew he was so versitile), the Dock Hag in Flapjack, the female Diamondhead and Fourarms from that What-If? Ben 10 episode, and I'll reiterate Cosmo since it's pretty surprising that a guy with such a deep voice could do Cosmo's high-pitched somewhat annoying voice. Even more surprising is that he's done plenty of anime voice work: Digimon Frontier (as Mercurymon), Cowboy Bebop (Cowboy Andy Von de Oniyate, Morgan, Vincent Volaju), DBZ (as King Cold) and was even main/secondary characters in Fist of the North Star (Rei) and the Bastard!! OVA (as Dark Schneider himself).
As for people that surprised me?
I've read somewhere that Brad Sherwood of Whose Line Is It Anyway? fame did a voice in Digimon Savers.
I was surprised that the voice of "Brave and the Bold" Batman was also a main character on the Drew Carey Show.
I was also surprised to learn that Michael Sisterniklass (voice of Dean Venture) did anime work.
When I was younger, I couldn't believe that Spongebob and Eduardo (Foster's) were the same guy. On the subject of Spongebob, Patrick was voiced by one of the titular "Gargoyles" (Broadway to be specific; I find the fact surprising since I've never seen Gargoyles and these shows have a different vibe). Also, I was shocked to learn that Mr. Krabs was Lex Luthor.
Greg Eagles (voice of Grim and Sperg in Billy and Mandy) is one of the Espada in Bleach.
One of the most surprising of all (which I should've recognized from the get go): Mel Brooks as Santa Claus in the Jimmy Neutron Christmas Special.
A more minor one: I was playing Destroy All Humans when Orthopox pops up. When he starts talking, I ask: "Wait, Zim is being competent in his plans for world domination?!"