Pushing Daises and Ugly Betty! I loved these two shows and then they decide to just cancel them! It's not like Hollywood has anymore great ideas anyways.
I really liked Pushing Daisies... mostly for Anna Friel (had a massive girl crush on her at the time), and that got cut short in my opinion :( Also a little English drama called Echo Beach. I miss Ugly Betty too, but it was getting quite stale to be fair.
Queer as Folk. If I remember right, the creators just decided to end the series because they'd done all they wanted to do with the story, so it doesn't really count as "cancelled", but whatever. I'm glad they didn't drag it out just for the sake of keeping it on air like a lot of shows do, but man, I miss that show. It was so, so, so good (even though I hated Justin and Mikey).
All the old-school Nick shows, especially Rocko's Modern Life, Aaahh!!! Real Monsters, Hey Arnold!, and Angry Beavers. I also miss Courage the Cowardly Dog. Now that was a fantastic show.
Also, The Powerpuff Girls. I don't care what anyone says. I still proudly watch that show on OnDemand to this day.
Drake & Josh, I miss that show so much; Invader Zim got cancelled, brought back, and cancelled again; and basically most of the shows from the old Nick and Disney channels....
Dark Angel, most definitely. Oh my god, I just rewatched it all recently, and raged. I forgot how angry I was. IT HAD SO MUCH POTENTIAL. It was way before its time. ;______;