Didn't know it was him, though he was less known at the time I reckon as he recently gathered a lot of fame due to his role in Mr. Robot. The Avatar franchise though has used quite some familiar voices throughout the years. Some of the more well known ones:
George Takei voiced a director of a prison for earthbenders during TLA season 1:
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Mako was voiced by David Faustino, who played the role of Bud Bundy in Married with Children:
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And Admiral Zhao was voiced by Lucius Malfoy (Jason Isaacs):
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There should be more, but these are the only ones that come to mind right now.