I recently discovered this map browser that lets you view historic county borders:
http://historical-county.newberry.org/website/USA/viewer.htm (http://historical-county.newberry.org/website/USA/viewer.htm)
You can set the date and zoom into whatever area you like. It has the option to turn on modern boundaries and county seats for reference. Names appear when you zoom in enough (about half a Midwest state). They've got info going back to the first colonies.
It's really cool to watch the checkerboard fill in and it gives you a sense of why some counties are shaped the way they are. Check it out.
Fascinating!
That's actually really interesting to me, because the Wikipedia article for my county says that it was formed from parts of 4 counties, and then itself was divided up twice. So I've always wanted to see what size it used to be.