http://michaelporath.com/projects/manifest-destiny/
141 maps progressing from the original 13 states to the statehood of Hawaii in 1959. Shows both territories and states. It's a bit difficult to follow what changes from map to map, but it's still an impressive presentation.
http://www.pershingms.org/MTT/index.html
This is a very helpful website. I suggest following the tour (although a few of the links lead to nowhere…) The painting "American Progress" and the map (scroll down a little) are interesting conversation starters.