This is NGC 3370. It's a spiral galaxy located some 100-million light-years away in the constellation Leo. Hubble's Advanced Camera for Surveys managed to capture this NGC 3370 in some truly remarkable detail. The galaxy is "face-on" which means it rotates on a plane perpendicular to that of the Milky Way (or, rather, to our perspective).
NGC 3370 has had a colorful history in astronomy. Back in 1994, NGC 3370 became the subject of serious scientific investigation when a type Ia supernova illuminated the region. This type of supernova is a standard candle which means measuring how dim it was allowed us to more accurately estimate how far away this galaxy is from us. This image in particular is sharp enough to allow astronomers to study Cepheids; another candle that allow astronomers to pin down accurate estimates of distance. These types of measurements allow astronomer to study the large scale structure of the universe such as the diameter of the observable universe, the speed at which galaxies move through space, and more accurate measurements for the speed of inflation.