While I like the choices that I've seen while reading these reply's, the show that I really miss is Studio 60.

The creator of the West Wing was behind the one as well. A sure hit...right?

NOT!!

Great actors, but poor story content plus a huge payroll doomed the show.

"Arrested Development" canceled after three seasons (two of which were shortened to 13 episodes)
"The Tick" The WB wanted to bring the series over to them, but FOX said no. It was a very funny show.
"Deadwood" why HBO let this one go is beyond me.

Well, I have a bit of an eccentric list I guess.

The Sentinel
Stargate SG-1
Star Trek: Deep Space Nine
Farscape
Due South
Grounded For Life
SeaQuest
Kids In The Hall
Sliders
Charmed
Tru Calling
Twin Peaks
Full House.

I know, a weird taste in shows, but these range from shows I still love and ended waaaay to soon.

Two other shows that were cancelled to soon were:

Day Break - There was 13 episodes of this show and only 7 of then were aired before the show got cancelled.

...and...

The Evidence - There were only 8 episodes in the show then it got cancelled.