Well, I can't think of any shows that jumped the shark, but there are a lot of shows that just declined badly.
Family Guy: I maintain to this very day that the first three seasons of Family Guy is some of the funniest stuff ever put on TV. When it was reborn it was still kinda good, but nowhere near the earlier seasons in quality. But the decline was extremely rapid and today I can't even watch it. It still has some funny jokes, hell, it still has some BRILLIANT jokes, but there's so much shit and rushed storytelling in every episode that I just can't be bothered to watch it.
American Dad: This one pains me, 'cause I used to absolutely love AD. It was almost like watching the old Family Guy, but with a lot of political jokes. Unfortunately AD has started to look a lot more like FG with random/weird jokes and poor storytelling. I'll give it a shot when it starts again in a few weeks, but if it doesn't pick up I'll probably stop watching.
Heroes: I gave up about 8 episodes into season 2. Loved the first season, but the second one felt like it was just dragging on forever.
My Name Is Earl: First two seasons were great. Season 3 started out pretty good, but it went downhill fast when Earl ended up in a coma. Season 4 was kinda good but suffered from making too many 'wink wink nudge nudge'-jokes to the audience. The show started feeling a lot less real and more like the characters knew they were just characters in a sitcom.