Frank Welker did an impersonation of his voice once, in the episode where Garfield gets the Hawaiian flu. There was this parrot that did impersonations, and impersonated Garfield. I feel that he could do Garfield's voice now, but apparently there is someone else. You might be able to see it on TV, but there's some commercial PSA about some writer's association featuring Garfield and Jim Davis. This was made recently, and it was a very good impersonation, though you can kinda tell it wasn't Lorenzo. But I digress.
Mark Hammil also did voices of 2 superheroes in one episode (The name escapes me at the moment, but the entire episode is on Youtube). Not to mention Greg Burson (sp? New Voice of Yogi Bear) also had a couple non-recurring characters.