I mainly play JRPGs, so "short" is not normally something I have to deal with. Most of my collection is between 25-60 hours long, but there are a few exceptions.
I managed to finish Vanquish in just over 5 hours, but I'm excusing it because it was such a awesome experience. It may have been short, but what was there was really well polished. All fun, no filler.
The second is Way Of The Samurai 3. This one is also excused since playing through the game once isn't an accomplishment when there's 22 endings. Each playthrough will take you anywhere from 1-3 hours depending on how much you screw around and which ending you get. Completing the entire game (including collecting 200+ swords and skills) can take anywhere from 90-300 hours, so it's only short if you only play through it once.