Though this course isn't specific to Northern Virginia history, these are some of the best and most useful online resources for Northern Virginia's history.
This free service offers old aerial images with very useful tools, like an overlay of current streets and a compare function so you can slide back and forth between years. For Northern Virginia, the imagery goes back 1949 so you can see what this region looked like before massive development. See an image you want to keep? You can download a hi-res TIFF for a reasonable fee.
Alexandria Building and Repair Permits Collection Index, 1896-1928
If you're researching property in Alexandria, this index from the Alexandria Library Special Collections is a must. Organized alphabetically by street name, the index has the property owner's name and the date of the permit. You still need to get the full permit from the library but this is a helpful and time saving head start. Be sure to check cross streets, too!
Fairfax County Court Historical Records Finding Aids
More than a dozen finding aids in PDF format make it easy to search indexes of early court records, including historical deeds, wills, chancery suits, marriage, birth and death records, and other materials that date to the mid-18th century.
Loudoun County Historic Maps
Hosted by Flickr, this collection of maps from Loudoun County's Office of Mapping has the classic 1853 Yardley-Taylor and new innovations incorporating historic data, like original land grant boundaries. One of my favorites is the rural delivery routes in 1926.
Library of Congress Map Collection
As part of American Memory pages, this site has dozens of maps of Northern Virginia. It can be searched by key word or browsed by locality but keep in mind that boundaries change so check for maps of adjacent counties and the District of Columbia. Super hi-res maps are available in JPEG2000 format or you can zoom in on the detail you need and then right click to save a smaller but still highly detailed file.
Virginia Chancery Records
This collection from the Library of Virginia is an incredible resource with thousands of pages of chancery court records from across the state, mostly from the 19th century. These fragile pages have been digitized and indexed so they are easily searchable. They can contain wills, plats, names of slaves and their owners, financial details, and information on how family members are related. Boundary lines changed so remember to check surrounding localities, too.
Friends of Freedmen's Cemetery Historical and Genealogical Resources
This site offers more than two dozen collections of 19th century records for African Americans in Alexandria. Transcribed from local and federal records, these collections provide insight into the lives of people of color living in and around Alexandria before, during and after the Civil War.