I can't believe no one has identified this fish yet. It is an endler's livebearer.
ERRR um, I think I mentioned Endlers right off the bat. Almost any feeder guppies you buy (at least in my area) show strong endlers characteristics if you let them grow up. I could get you a thousand just like the one pictured tomorrow and buy them for $1-$2 a dozen. My understanding with endlers enthusiasts was that if there was any chance they had been around guppies and possibly crossbred then they weren't wanted. I have a tank full of the cute little buggers now, and would love to sort the males out and get rid of them, because they don't grow big enough to be good feeders for my cichlids.
The majority of mine are classic coloration and characteristic, but they have been in with guppies for some time so I don't get excited.
Dave