I'm not going out of step with the rest of this thread. I want Firefly back as well.
Other than that, I'm hard pressed to think of many. Harvey Birdman: Attorney at Law was already mentioned.
I'm fairly lucky though, as two of the ones that were dead for different amounts of time have come back or are coming back. I speak, of course, of Futurama and Eastbound & Down.
I would add Flight of the Conchords to that list, but from what I heard, they were running out of material into the second season, and decided to call it quits. If they ever had a creative "second wind" and came back though, I'd love it.