Here via musesfool. My second and third etcs would be ones that other people have mentioned. Farscape, because we should have had a full fifth season. And in a similar vein, Firefly of course.

If it weren't idiotic, a third season of Dark Angel would have been lovely, although, in most of the scenarios that I could envisage, it would have been idotic.

Word on others' suggestions of Cupid and Dead Like Me and I'm sure there are others that I could think of, and will remember about five minutes having gone to bed.