LOC's Digital Resources - contains thousands of maps and photographs in the public domain
Chronicling America - resource for historical newspapers
In Custodia Legis - LOC's Law Library blog covering historical topics and figures
Northwestern University's Knight Lab - TimelineJS
TimelineJS is an open-source tool that enables anyone to build visually rich, interactive timelines. Beginners can create a timeline using nothing more than a Google spreadsheet.
StoryMapJS is a free tool to help you tell stories on the web that highlight the locations of a series of events.
History of the Federal Judiciary - Resources that may be particularly useful for court history projects.