From the minimalist Windows Phone Store description:
A world map that shows the regions of day light and night.
It updates once every minute.
Date and time of day can be selected.
As with all such map projections, take the relative sizes of the countries and land masses with a pinch of salt - it turns out that the Earth really is round - but having it laid out in 2D does help work out which places can currently see the sun. Attempts to map this onto a 3D-rendered globe usually fall foul of 'not being able to see well near the edges'.
Here's the somewhat clumsily-named 'Day / Night Map' in action:
In fact, in addition to the visibility (sic) of where it's day and where it's night, there's also here the rather useful facility to explore sunrise and sunset times for any day of the year. Yes, there are calculators for this sort of thing elsewhere, but Day / Night Map is a useful utility to bring these functions together on your phone.
There's a 30 day free trial in the Store here.