Remember Rob Reiner's appearance on The Odd Couple?
He played Myrna's boyfriend Sheldn. (the O was left off his birth certificate)
Of course Myrna was none other than Penny Marshall. Reiner's then wife.

http://prodtn.cafepress.com/3/12245143_F_tn.jpg
The one and only Sheldn t-shirt. (http://www.cafeshops.com/theoddcouple.12245143)

Reiner also made appearances on The Partridge Family and Room 222.
Any others?

He had a small part on one episode of "The Andy Griffith Show." It was the one where Goober was having some kind of promotional giveaway at the gas station, and the tickets were printed wrong. Meathead is shown at the beginning, working at the print shop.

He also appaered in an episode of The Rockford Files titled"The No Cut Contract",where he played a con artist named Larry King Stutervant.

Meathead for governor of California in 2006! Terminate Arnold!