The mysterious Red-Headed Gardner of the Gowanus, who works a patch on the east side of the Union St. bridge, has a nice spread going on right now. I went over to look for pollinators, and saw five different species. These were the best shots: Some of the sweat bees have this metalic color, but all over; on this one it was just the thorax.
You guess is as as good as mine. I've yet to find a really good on-line identification guide. More than forty species have been found in NYC gardens.