Also, I realise that not all of these shows had a "Jump The Shark" moment, it's just easier to put that as the title.
My Little Pony: Friendship is Magic: I think that the show is going downhill, as of A Canterlot Wedding. A Canterlot Wedding is the most overrated episode in the entire series, in my opinion- the writing was merely average, the pacing was off, and the villain, while having a catchy song, was DUMB and boring to watch.
Spoiler: show
Two and A Half Men: It was already starting to rot and this show finally jumped the shark after they replaced Charlie Sheen. 'Nuff said.
The Big Bang Theory: This show was fantastic and fresh until Season 4. In seasons 1, 2, and 3, characters not only had more depth, but the jokes were funnier. The show is now about the romance between the guys and their girlfriends with occasional poking-at-Raj thrown in. Sheldon has been Flanderized horribly(remember when he gave Penny a hug? They wouldn't make an episode like that today). The scenes with the three girls chatting together are boring and unnessicary. It used to be about four scientists, cracking science jokes, which made it uniquely nerdy. But now it barely has that kind of thing any more, and is a generic sitcom made to appeal to the masses. I only put it on as background noise now. There is rarely a good episode, it's only just watchable.
Spongebob Squarepants: Got slightly worse during Season 3, awesome again during The Movie, then turned bad after it was un-cancelled. I just did a marathon of seasons 1-3 and the movie. I tried watching Season 4, but I just couldn't get past Good Neighbours. There are occasionally REALLY great episodes, and most of them are watchable(errrrghhhh... The Splinter...), but the characters are now overly insensitive and stupid. It was part of what made the first three seasons great, but it shouldn't be the basis of the series.