[Bonnie]

I can't believe no one mentioned these! That just makes me feel old... Other than Firefly, or shows that have since been brought back (like Futurama, Arrested Dev, etc.) these are the most famous shows for getting screwed over before their end:

Deadwood - best western series ever
Carnivale - one of the oddest shows ever
Twin Peaks (It's been 20+ years for this one though, so it's just wishful thinking)

Oh, and Good Eats, Secret World of Alex Mack, Adventures of Pete & Pete, and Space Cases, the last three you'd have to reboot with new actors but it'd be totally doable.