Don't forget The Happy tree friends!
The Grim Adventures of Billy and Mandy have slowly grown on me. As did Courage the Cowardly Dog. Unfortuantely, they don't show Courage too much anymore.
I know this isn't a television show, but I figured I'd mention City of Rott, it being one of the most violent and gory horror cartoon movies I have ever seen.......Zombies everywhere and a crotchity old man with a walker to fend them off