Here are some shows and their memorable "jump the shark" moments:
The Brady Bunch - It was all pretty much over when Cousin Oliver inexplicably joined the Brady clan. It's a sign of any sitcom's demise when a young, precocious kid is brought on board in a desperate attempt to boost the ratings after the other kids on the show grow up.
Dallas - Bobby Ewing was dead, then he mysteriously showed up in a shower -- the cliffhanger revolution turned out to be one of the dumbest plot points in serial TV history when it was revealed that the entire season before in which Bobby had apparently been killed was just a bad dream by Bobby's wife Pam. The show never recovered creatively.
The Dukes of Hazzard - Bo and Luke Duke left to pursue careers as NASCAR drivers (while the actors who played them, John Schneider and Tom Wopat, left the show over bitter contract disputes) and their never-before-mentioned doppelganger cousins Coy and Vance took over the misadventures in Hazzard County. Bo and Luke eventually came back, but the damage was done.
Heroes - The great first season was quickly ruined when the time-and-space-controlling Hiro traveled back to feudal Japan in one of the most boring and ill-conceived storylines ever written, grinding the momentum of the hit series to an abrupt halt.
Laverne and Shirley - When the odd couple roommates left Milwaukee for Hollywood, they must have forgotten to pack their charm and comic timing, because the episodes that followed were horrifically bad.
Lost - Once the Smoke Monster killed my favorite character, Mr. Eko, the show died a bit for me.
Roseanne - The blue-collar comedy definitely jumped the shark when the Conner family struck it rich by winning the lottery.
Smallville - Although it still had a few moments afterwards, when Clark Kent finally moved to Metropolis it felt like something was missing. The fact that he was pretending to be some vigilante named the Blur instead of Superman was the deathblow.
Star Trek - The franchise defied the odds and continued to live on in various incarnations, but the original series began to crumble with the silly episode in which some alien chick stole Mr. Spock's brain (I kid you not).
Supernatural - I really liked this show, even when the mythology evolved beyond the monster-of-the-week premise to an epic Heaven vs. Hell storyline, but the recent plot about angel Castiel "becoming God" finally made me give up.
Three's Company - Success often ruins a man. The popular sitcom survived the departure of Suzanne Somers, but finally fizzled when Jack Tripper stopped being the struggling chef-wannabe living with two attractive women to make ends meet and become the owner of his own bistro.
V - I think the moment that really made the original science fiction show steer off course is when the alien baby, Elizabeth, started growing up at an accelerated rate with no explanation and then miraculously stopped when she become a hot twenty-something-year-old.
The X-Files - The final years were a mess, but the nail in the coffin was the pointless plotlines that were developed for the first movie adaptation that just muddied the waters without offering any satisfying resolutions.
If there are any other shows that you loved that jumped the shark that I haven't mentioned, please let me know.