Above is the world's smallest known vertebrate, Paedophryne amanuensis. Researchers found the frog in 2010 in southern Papua New Guinea and just announced the discovery yesterday. From National Geographic:
"World's Smallest Frog Found—Fly-Size Beast Is Tiniest Vertebrate"
Scientists locate the teensy animals by listening for their calls and trying to zero in on the sources of the sounds—no mean feat, since the high pitch of the calls make their sources especially hard for human hearing to locate.
(Louisiana State University biologist Christopher) Austin and graduate student Eric Rittmeyer tried four times to find the frogs before exasperatedly grabbing a big handful of leaf litter and putting it in a plastic bag.
The scientists then combed through the contents until "eventually we saw this tiny thing hop off one of the leaves," Austin said.