Alright here are some of my "Jump the Shark" moments for some television series that I have seen. If you something better then tell me.
Simpsons- The Simpsons jumped the shark probably in my opinion during the late 90's when they aired the episode with Skinner not being the real Skinner, it was a total blow to all Simpsons fans who had relished Seymour Skinner and his relationship with his mother and now he wasn't the real Skinner. It got a bit idiotic and completely changed the edge of the Simpsons since. It hasn't been the same since it's hey-day of the 90's.
Cosby Show- Addition of Cousin Pam- One of the dumbest moves I think any television program has ever done was when the Cosby Show added the character of Cousin Pam to the series. Not only did she not add anything, she didn't standout and seemed like a cheap move to bring in a new character for a show that sagging in ratings.
Heroes- Odd twist here, but I think Heroes Season 2 in itself Jumped the Shark and just killed the series. The first season was good, not great as some Heroes nerds point out, but after the final episode of the Season, the expectations died off in the Second Season as the characters were forced into boring storylines, un-interesting events, and lack of coherence as well as good acting. The show tried a CW approach with Claire's relationship, tried making the nerd scientist Mohinder bad, blah, blah.....The show hasn't recovered since that dreaded Season.
Family Matters- Urkel became to annoying. It was good in the first few years but when Urkel started inventing all these weird inventions the show went into the realm of absurdity especially when Stefan was created, you almost questioned why they needed Urkel to begin with.
American Idol- I know I put it up earlier that Paula Abdul leaving could impact the show, but in my opinion the Jump the Shark moment is Kris Allen beating Adam Lambert. Perhaps Lambert's homosexuality played a role in him not winning? Clearly though he had the better voice.
Rurouni Kenshin- Post-Shishio saga, the show never recovered after that incredible saga and seemed to rely on just a bunch of mini-arcs to try to preserve the show but at around episode 95 the show finally ended, proving how the last 30 episodes didn't stack up to the Shishio saga whatsoever.
Death Note- I love Death Note and still give it a 10/10 however after Episode 25......SPOILER BELOW......................................................................................................................
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when L dies, the show lost a sense of it's essence, I see what they were trying to do, by making the show continue onward into the future they would have to get rid of L however, L's replacements Near and Mellow weren't nearly as interesting as their predecessor or complex. Losing L really was a bad move on the Death Note creators, but atleast there were only 12-13 episodes after his death or so.
Well that's it for now, if you got more please list them.