possibly the coolest specimen I ever kept was a fiddler crab. He was adapted to underwater conditions, and oh man he was a freakin' riot! I may have lost a few neons to his hunting techniques (waving the larger claw in the air, waiting for a fish to swim into it!), and he certainly hogged all the food that made it to the bottom, but he was easily the most interesting fish/crustaceon I've ever kept. If you happen to see one in a LFS, I would say buy it and watch closely!
BTW, he ended his poor little life by crawling out of the tank and being attacked by the unsuspecting cat! That was a showdown for the ages to be sure.