I am just so thrilled with this site, that its worth yet another mention. Most genealogists are familiar with the "must have" book, Map Guide to the US Federal Census, by William Thorndale and William Dollarhide - you just can't do census research without it! But recently I found a website, hosted by the Newberry Library (Chicago), that really is incredible. Not only does it show historical county boundaries, but you can search every year, not just the census years. From the "Atlas" main page, at
http://www.newberry.org/ahcbp/state_index.html, pick the state you're interested in (although unfortunately data isn't available for Georgia and Tennessee at this point) and then from the map that opens, choose the dates that you're interested in. Just like we're used to, the current county boundaries are visible in the background, with the boundaries of the dates chosen are in black. You can zoom in and out which is really helpful in states like Texas that have something close to a zillion counties.
Another really neat thing is back from the main page, underneath each state's map link is the word "metadata". If you click on that, you can view all of the documentation that goes along with each map, including commentary, bibliography, and the all-important "preferred style citation" information, so that you can easily and properly cite your source!
I've "tweeted" about this already a couple of times (I'm "rcurious", in case you tweet too), but I think its just such a great genealogy tool. The Newberry Library has some other way cool gems for genealogists, but I'll save that for another blog!