The f-word has been said twice on Saturday Night Live, but I think they’ve been censored in re-runs. The second time was said by Norm MacDonald, and I don’t think it was censored in the original live broadcast.
I’m not sure how that compares to other shows, though. Yes, it’s on network television, but it’s live so it might not fall under the same restrictions.
I’ve heard “shit” on ABC’s Once and Again, airing at 10pm. It was said by a character in one of the black-and-white monologue scenes, and after she said it, she gave this guilty look like she knew she wasn’t allowed to say it.
Lego:
Motherfucker.
Nope, during the live broadcast, Norm said (under his breath) “What the fuck what that?” during Weekend Update.
Classic.
Also thought’d I’d add…
Paul Shaffer mistakingly said it instead of “fluck” on March 15, 1980 (100th Anniversary Show) and Charles Rocket exclaimed, " … who the fuck did it?," on February 21, 1981 (Host: Charlene Tilton) and of course the Norm MacDonald one on April 12, 1997 (Host: Rob Lowe)
The commercials for this bill it as “a first in television history.” However, since there have been all these occurances, I’m going to sue Comedy Central for false advertising, fraud, and wrongful death. Well, maybe not that last one. 
I remember one Good Morning America show which was talking about two boys who had a horrible relationship with their parents. They showed a clip from the camera in the house and almost every other word was bleeped! Man that was funny. I don’t really understand the bleeping because you almost always know what they are saying you just hear shh instead of shit 
hehe.