For teachers or students that would like to compare different types of maps, there is the site GE Teach which allows you to select from gallery of map layers to compare side-by-side. In addition to comparing maps, there are also drawing and measuring tools that can be used.
GE Teach provides a great way for students to compare data sets in a map environment. The example above compares a political map with a topographical one. Great use for history, geography, or even science.