Sticking to once good or great shows that have run out of steam (rather than shows that sucked then and suck now), I'd say The Simpsons. They had an amazing run for about a decade and the last few years have either gotten better or I've learned to adjust my expectations than most of the episodes from the early to mid- 2000's but it's still time for them to wrap it up.
Those two are the most obvious examples. I can say that "Chuck" made the right decision by making this upcoming season the last ahead of time. "House" should probably make this upcoming season the end, too. "How I Met Your Mother" is the biggest one that needs to pick an end date ASAP and stick to it. The last couple seasons have had way too much filler as the prolong the inevitable indefinitely.
30 Rock - the show has been running on fumes the last couple seasons, and even more apparent last season, where the jokes just seemed recycled.
Weeds - this show can take a page or two out of Breaking Bad, and figure out an end date. I just finished watching Season 6 on DVD, and it was just unwatchable, to a point where I wish all the characters would die!
I am a picky television watcher. There are some shows that I just LOVE when they first come out and can't get enough of (Glee, Big Bang Theory) but then I sort of lose interest by third season. I have maybe a handful of shows that I actually watch regularly and still enjoy (Vampire Diaries, Community, Raising Hope, Project Runway, Survivor). The Walking Dead while only having 6 episodes in their first season has me intrigued. I hope they don't run out of steam, especially with all the great stuff they have to go on from the comics. Shows that got old for me fast: How I Met Your Mother (Just can't take it anymore) Glee ( I still enjoy the musical numbers, but the heart of the show is just gone) Big Bang Theory (Still funny at times, but just not holding my attention any more) Biggest Loser (Losing weight I get the point) Justified (I thought Timothy Olyphant would be enough to keep going with this one...not so much) Pretty Little Liars (watch this one with the teenager, but it drags and I just want them all to die) Terra Nova (bored after the second episode)