My Name Is Earl.
I couldn't believe it when it was cancelled. The first, second and fourth seasons were all excellent, and fourth series ended on a cliffhanger too.
Granted, it had lost a lot of viewers due to the mediocre third season but it was redeeming itself with season four before Fox (I think) cancelled it for no particular reason.
After it was cancelled, the guy who created it started Rasing Hope instead, which isn't half as good.
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