There is a variety of information, including some state census records (including Iowa, Illinois, Minnesota, and Wisconsin), military officers from the 1820s-1830s, the Compendium of the War of the Rebellion, Postal Databases, and more. Not all are indexed, but images are browsable.
A complete list of HistoryKat's content can be seen here.
The price for an annual subscription is very reasonable---currently $24.95. Browse their content list to see what they have. I've known the owners for some time and their site fills a nice niche at a reasonable price.