From Memorial Day 2007 (yesterday) until D-Day, June 6, 2007, the popular web site ancestry.com is making all military records open to the public for free. They boast that their collection has a complete list of WWI Draft Registration Cards, WWII Army Enlistment Records, as well as a Civil War collection, and an array of records stretching back to the Revolutionary War. There are also a few surprising gems such as facsimile reproductions of the military newspaper Stars and Stripes from both WWI and WWII. The collections are heavily weighted toward the U.S. Army, with very few for the other branches of the service. (A full list of record groups is available here.)
The web site does require registration to view the records, and the installation of a special plug-in if you wish to view facsimile reproductions that are available for some records.
While the collection is useful for looking up records of individuals, alas there are no advanced statistical tools, or other tools that will allow the user to glean aggregate data from the site. That said, however, the collection should be commended for its wide array of records.