Moonphase App with over 90000 downloads worldwide. This Moonphase App is one of the most popular moonphase apps around.
With a single swipe you can see the current, past and future moonphase in high-res quality. Also share it via Facebook, email or SMS, so all your friends will know what moonphase it is.
For android users, there is also a nice widget, so you can see the current moonphase on your homescreen as well.