I got a few, here are the ones I can think of so far:
1- While watching my brother play Tales of Symphonia for the first time, I couldn't help but think that Lloyd's voice sounded familiar... then it hit me: it's the same guy who voiced Robin in the Teen Titans cartoon!
2- I was eager for the Bleach cartoon to reach the States, especially to see who was playing the role of Ichigo, someone by the name of Johnny Yong Bosch. One morning, I happened to catch a rerun of Power Rangers on TV, and his name was in the credits. Cue the double-take.
3- Finally, apparently Eeyore is voiced by Peter Cullen, better known as the voice of Optimus Prime. Weird, huh?
EDIT: More revelations!
-I got into Freakazaoid after they stopped airing it, and quickly recognized the voice of Dexter Douglas (that is, Freakazaoid's hapless alter-ego) as one Danny Phantom. Go figure, huh?
-Also, I was surprised the learn that Steve Blum of all people voiced Guilmon in the English version of Digimon Tamers kinda different when compared to his usual roles...
-Ever watch the movie HOOK (Y'know, with Robin Williams and Dustin Hoffman? Turns out the kid who played him would later go on to voice characters like Jake Long and Prince Zuko... wow...