The Grim Adventures of Billy & Mandy, on Cartoon Network
BCDB; Cartoon Network Wiki; IMDb; TV Tropes; Wikia; Wikipedia
See also What a Cartoon!
This started out as "Grim & Evil," which was actually a combination of two shorts. The "Evil" part was Evil Con Carne (IMDb; Wikipedia). It was about this guy named Hector Con Carne, whose brain was preserved in a jar that was like attached to the top of the head of a bear named Boskov. Hector wanted to take over the world, and he had a remote island with a little army, and his top people were General Skarr, who hated him and wanted to take over for himself, and a scientist named Major Dr. Ghastly, who loved Hector. Eventually I guess "Grim & Evil" sort of spun off into two separate series, but I never really cared about "Evil" quite as much as I did about "Grim." Which itself was never that important to me, either....
But anyway, the premise of this show is, there are these two kids, Billy, who is really stupid, and Mandy, who pretty much just hates everything. They won some game or whatever, I forget exactly, but the upshot is they beat the Grim Reaper, who now has to be their best friend like, forever. And most of the time he really doesn't seem to like this, but he has no choice. So now they have all these wacky supernatural adventures.
Anyway, I watch it very rarely, but it's not so bad. I think Mandy's cool and funny, anyway. And I feel bad for Grim. And he's funny. He talks with a Jamaican accent for some reason. I don't much care for Billy, though. Oh, also Billy has a friend named Irwin who has a crush on Mandy, but she can't stand him. I can't think what else to say, except this one time Mandy had to sing "Over the Rainbow," and it was the best thing ever. Also there was a crossover one time with Codename: Kids Next Door.