Glad I came upon this topic. As I have some myself:
First off it Transformers G1. In all honesty, I didn't know that Peter Cullen voiced both Optimus prime and Ironhide until I checked it out on Wikipedia. Ditto with Michael McConnohie and his portrayal of Track and Cosmos.
I was also surprised when I discover that Charlie Adler did Starscream in the live action movie. I normally associate him with higher pitched (yet raspy) characters.
I was even more surprised when I discovered that Rattrap, Dinobot and Waspinator from Beast Wars had the same voice as well. And most of us know who he is too (For those who don't, it Scott McNeil).
Some actors like Frank Welker, Mel Blanc, Billy West and them are self explanitory Though I must admit, None of West's characters on Futurama even sounded the same. I also must say that he does a pretty good Phil Hartman impersonation.
And at times, I honestly think that Kari Whalgren is impersonating Tara Strong and Grey DeLisle at times.