The digital map collection of the Perry-Castañeda library (University Texas) is a free map download source. For many countries you can get map data right online. Besides current topographic maps, the virtual library provides historical and thematic maps as well.
If the content of the Perry-Castañeda library map collection shouldn’t suffice, here you get to a map collection linklist overview.
Here you go: http://www.lib.utexas.edu/maps/index.html