A little bit of science fun:

Scientists have discovered a new species, a member of the carp family called Paedocypris progenetica, that could claim the title of world's smallest vertebrate. Less than 1cm long when fully grown, these awesome little buggers live in acid pools, possess unusual grasping fins, and have reduced head skeletons that leave their brains unprotected by bone.

Neat-o.

UPDATE: In my ongoing effort to prove that I am the world's nerdiest vertibrate, I spent my lunch hour reading up on this a bit more. It turns out that there is at least one other species that can challenge Paedocypris progenetica for the title of smallest vertibrate: the male anglerfish (Photocorynus spiniceps to his friends), measured at only 6.2mm in length.

However, there can be some difficulty in accurately measuring the male anglerfish, and in determining when he has reached maturity, because he is what is known as a sexual parasite. He lives attached to his ladylove, who is up to half a million times larger than he, and lets her take care of all the swimming and eating while he concerns himself exclusively with aiding in reproduction. This may make him sound like a world-class freeloader, but we should cut him some slack because he is slightly hampered by “testes so huge they nearly fill his entire body cavity, crowding his other internal organs.” (Refs)