USGenWeb
Unlike FamilySearch, USGenWeb lacks a straightforward search feature that allows you to just enter family names and press the find button. Instead, you go to the home page, choose the state you're interested in, and then you're directed to a website that is centered on that state. This website will then have all manner of records available, often broken down by individual counties.
It’s a completely free project, run by volunteers. This implies that the quality of each state's website varies; some offer a large number of digitized, easily readable census records and military files, whilst other sites are strewn with broken connections and outdated graphics. You can’t complain, as it’s a free resource, but it does mean that it’s not necessarily going to be easy to find relevant information on the site.
It is a valuable tool for mapping more broad historical trends, though. This is a priceless tool, for instance, if you wanted to examine household sizes, common names, or occupations. This also is a really helpful tool for anyone who wishes to compare their ancestors' lifestyles with current trends or is interested in local history in general.
Website: usgenweb.org