Foster's Home for Imaginary Friends, on Cartoon Network
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Created by Craig McCracken, who also created The Powerpuff Girls. There's this 8-year-old boy named Mac, whose mom says he's too old for his imaginary friend, Bloo (short for Blooregard Q. Kazoo). The really cool thing is that everyone can see and hear imaginary friends. So they're real, and all "imaginary" really means is that they were created by the imaginations of little kids. Anyway, there's this foster home for abandoned imaginary friends, which was founded by this little old lady named Madame Foster. It's run by her granddaughter, Frankie (who's pretty cool), and Madame Foster's own imaginary friend, an officious old rabbit with an English accent, named Mr. Herriman, who she imagined as a little girl, but never outgrew (he's actually Frankie's boss, very demanding, with ridiculously exacting standards). There are several other imaginary friends who become friends with Bloo and Mac when Bloo comes to live there, including Wilt (who is a really tall, red, basketball-playing sort of rag doll or something; and also overly polite, apologetic, and accomodating), Eduardo (who is scared of most things, even though he looks like a cross between a bull and a chupacabra, or something; Bloo often teases him for being a cry-baby), and Coco (sort of a cross between a bird and a plane, who only says "coco," though everyone seems to understand her anyway; and she lays plastic eggs with useful surprises inside). There's also an imaginary friend named Duchess, who everyone hates, because she's so snobby and mean (but she's rarely seen in the series).
I loved the pilot movie; it was one of the most deliciously zany and hilarious things I've ever seen, kind of like The Muppet Show, or something. What happens is, Duchess hatches a plot with Mac's older brother, Terrence, to get rid of Bloo forever. Which Mac helps thwart, and therefore Madame Foster allows Bloo to stay forever without getting adopted (as adoption is the purpose of the Home, of course), as long as Mac promises to visit every day. And that's the premise of the series. Mac and Bloo hanging out with the other imaginary friends, doing... whatever.
The show has really got some very clever gags. It's pretty retro, from the Victorian mansion it's set in, to the animation, to the theme music (which fits the show quite well, I think; it puts me in mind of some kind of old-fashioned amusement park, or something). So I kept on loving the show for some time after the pilot. But after awhile, it did start to get a little old. I still like it, and I'll always love the whole concept, and some eps can still be really good. But not always. And fairly often, I'm not even sure why Mac and Bloo are friends. Bloo can be pretty funny, but he can also be totally conceited and self-centered. He often annoys his friends, and rarely shows much interest in anyone beyond the main characters (there are a number of other recurring imaginary friends, as well as one-shot characters), except when they can be of use to him, or are at least willing to pay attention to him, cuz he craves attention. Still, the show's funny enough, even if Bloo isn't always likable.
And I guess that's about all I can think to say, for now. But there have also been a couple of specials, Good Wilt Hunting and Destination Imagination.