TV blog Warming Glow points to this clip from the upcoming IFC series Comedy Bang! Bang!, based on the popular, Scott Aukerman-hosted podcast of the same name. For the TV version, reportedly a surrealist take on the late-night talk show, Aukerman is joined by bandleader and "one-man musical mastermind" Reggie Watts, a performer brilliant enough that he could make the show worth watching by himself. He also provides the opportunity to, say, have Jon Hamm rap about his love for the bygone TV series Taxi.
A couple notes. 1) Jon Hamm, as he has proven on Saturday Night Live, 30 Rock, and elsewhere is a funny guy—and he really likes Taxi. 2) Reggie Watts knows a freestyling highlight when he hears one: "Tony Danza's there!" was definitely Hamm's best moment, and the line worth looping.
But to see Hamm at his freestyling best, I recommend the clip below, also noted by Warming Glow. This one is from the Comedy Bang! Bang! podcast, and while host Scott Aukerman gets it off to a fairly rough start, Hamm saves it with his excellent chorus, and then the great comedian Paul F. Tompkins brings it home with his take on a rapping Garry Marshall. ("A lot of people say to me, 'It's autumn! Where'd you get those sweater vests?' I say, 'I bought 'em!'") Note, too, the "turn my headphones up" at the beginning. And enjoy.