Josh Robert Thompson (above) is one of the better impressionists around, although he is usually heard (most notably as "Arnold Schwarzenegger" on the Howard Stern Show) and seldom seen. Didn't know, until now, that he has appeared on Late Late Show with Craig Ferguson, and has his own YouTube channel.
Here are two funny clips as Schwarzenegger - prank-calling Walter Cronkite, and (from Stern's show) George Takei.
In addition to Schwarzenegger, Thompson also does a pretty good Morgan Freeman.