Frasier
Two and 1/2 Men
From the second season, the theme tune to sitcom Birds of a Feather was sung (not very well, but endearingly) by the main two characters.
Is it Mila Kunis? Remember she wasn’t Family Guy cast until a little ways in (Lacey Chabert was the voice of Meg to start).
“South Park”.
I think Hee Haw’s theme was sung by at least Buck and Roy if not the whole cast.
I’d say it counts if and only if the show’s voice actors (hence the emphasis added above) recorded the theme.
By that standard, Jim Henson, Frank Oz, et al., really did sing the theme song.
Closing and he sure did. Good job, too.
Yep.
Did they? I know this makes me look clueless, but I had no idea they sang it and no idea it changed when Joel switched to Mike. I know the words changed, but not the singer and I had no idea it was them.
The Bill Cosby Show (the 1969-71 one, not to be confused with The Cosby Show) featured Cosby’s dulcet tones on the Quincy Jones-composed theme “Hikky Burr.”
Biffy, you beat me to it.
How about the Smothers Brothers Show? Not the variety show, but the half-hour sitcom that preceded it.
It has been since season 4, when Kunis joined the cast. The theme song has been re-recorded several times. One notable time was when viewers thought that Stewie was singing “effin’ cry” for “laugh and cry.” MacFarlane re-recorded the line so that “laugh and cry” sounded more distinct.
If closing themes count, there’s “Long Lonesome Highway” from Then Came Bronson, sung by star Michael Parks. The song also became a Top 40 hit at the time.
I recall reading that Two and 1/2 Men wasn’t actually performed by the actors.
Didn’t Chuck Norris sing the theme for Walker Texas Ranger?
It isn’t. It’s actually studio musicians. The actors merely lip-sync the words. One of the singers is Elizabeth Daly, who plays Tommy on *Rugrats. *
The theme song or the show itself?
For sufficiently generous definitions of “sung”, I nominate Gilligan’s Planet, an animated sequel to the live-action series. They got the entire cast, except for Tina Louise. Dawn Wells did the voices for both Mary Ann and Ginger.
Actually, looking at animated-series intros from the 1960s and 70s, having the voice actors sing and/or exposit the premise looks fairly common.
I thought of another one.
It was a 1965-1966 sitcom starring Tom and Dick. They sang the theme for half the show’s episodes.
See post 50.
Ya ninja’d me, ya dirty so-and-so!
And Cartoonacy ninja’d YOU! 
Interesting speculation follows: The angel brother (Tommy, IIRC) was apparently being supervised by an unseen stentorian-voiced Heavenly functionary whom he addressed as “Ralph.”
Fast-forwarding thirteen years to Mork & Mindy, Mork made a weekly report to an unseen stentorian-voiced Orkan supervisor whom he addressed as “Orson.”
I swear I saw his name in the credits being given as “Ralph Orson” (although imdb is apparently trying to convince me that I hallucinated that, and he was only ever credited as "Ralph James). I do remember the stentorian voices being similar, though.* Is it possible the same voice actor provided both?
Here’s one nobody else did yet: The Heights a Melrose Place spinoff (which is to say, a Beverly Hills, 90210 ripoff :D) took on as a theme song How Do You Talk to an Angel, supposedly performed by the eponymous pop band “The Heights.” I understand that the lead vocal was provided by the actor who played the lead singer in the group.
*considering that “stentorian” is kind of a specific vocal quality, of course, I suppose it would be odd if they didn’t sound similar.
- shakes fist in impotent rage.