Hamill already had a career in cartoon voices well before he strapped on a lightsaber. He can be heard in several early 70's Hanna Barbera shows.
He also has a small role in Ralph Bakshi's Wizards, which might have had a longer run if another Fox summer '77 release hadn't gone through the roof. ;)
Jon Stewart noted the resemblance of one of disgraced Illinois governor Blagojevich's lawyers to that of Palpatine earlier this week...