Florida's Educational Technology Clearinghouse has a collection of more than 5,000 historical maps. The maps are licensed for free download and reuse by teachers and students. The collection is organized by continent and country. The US category is further broken down and organized by state and by historical theme.
Thanks to Donna Murray for the link.
Applications for Education
Historical maps can be very useful in helping students to get a better sense of an entire historical event or story. These maps could be used as a supplement to a history lesson or as the beginning of an inquiry-based lesson. These maps could also be used as overlay images in Google Earth files that students develop.
Here are some related items that may be of interest to you:
Free 33 Page Guide - Google for Teachers
Historic Maps in K-12 Classrooms
Maps of War - Animated Thematic Maps