Short but awesome huh? Well, as someone else has already mentioned, Afro Samurai, which, in my personal opinion, is a 5 episode + 1 movie-I-really-need-to-get-around-to-seeing instructional video on how to be awesome.What else? Kannazuki No Miko, a rather lovely romance story, with giant robots, I know a mecha hater (as opposed to yours truly, a "Mecha-dork", I just love mechs, I'm not particularly picky about the size, see: Appleseed) who got some enjoyment out of that one, but I'll admit it's not for everyone, Wikipedia first. The 24 long Chrono Crusade also makes the grade as far as I'm concerned, then again I never read the mangas and was therefore never scorned by the anime, then again it did have a christmas episode...
Gundam fans would be doing themselves a disservice if they didn't watch Mobile Suit Gundam 0083: Stardust Memory, it's only 13 episodes long, watch it.
Oh yeah and The Melancholy of Haruhi Suzumiya, but you probably already knew that.