Maps, maps, maps. You gotta have maps. The two below are actually wall paper; we purchased them from nationalgeographic.com and asked a carpenter from our church to put a wood frame around them.
Click here to see the National Geographic maps.
The first picture is of a map we actually painted onto the wall. We used the "overhead projector trick" to accomplish this. We simply photocopied a map, put it on overhead film, projected it onto the wall, outlined the countries with Sharpie markers, and painted them in with paint. The border I purchased on clearance at a wallpaper store. We estimate the whole thing cost us around $8.
The map of the world was put up the year we studied world history, the map of Europe (below) was added two years later when we were studying Anabaptist history.
And this isn't a map, but, just for fun, I have always wanted to put something like this Monet mural in my classroom. This particular one was in my living room instead. Maybe someday . . . .