AncestorSearch using Google Custom Search: searches using the full power of Google, and then filters exceptionally well so you're more likely to find mentions of ancestors you otherwise might miss buried in thousands of Google search results. Many people are surprised at what AncestorSearch finds, even for ancestors that they had searched for exhaustively over the years.
Historical U.S. County Boundary Maps interactive tool: find the correct U.S. jurisdictions for the place and historical timeframe you're researching. Then, overlay research locations on the map such as courthouses, cemeteries, churches, and libraries, and link right to them for more information.
Historical World Political Boundary Maps interactive tool: find the correct world country boundaries for the place and historical timeframe you're researching
I could have used all three of these multiple times in my classes this past few months. Take a look and see what you think.