District of Columbia
District of Columbia Government -
District of Columbia Real Estate Assessments - Tax records, including sale histories

General
Neighborhood Views from Washington Post -

Maryland
Montgomery County Tax Assessments - Tax records and property assessments

Schools, Crime, Demographics
Census Data by Zip Code -
CrimeReports.com -
DC Crime -
Demographic Data -
GreatSchools.net -
Megans Law - Registry of sexual offenders
MyLocalCrime - Crime Stats
Reseach a Community - Onboard Navigator
School Matters -
Sperling's Best Places - Sperling's Best Places

Transportation & Traffic
Dulles/Tysons Metrorail - More on the planned Dulles/Tysons metro
Dulles/Tysons Metrorail (Silver Line) - Dulles/Tysons Metro Updates from Wikipedia
MARC Trains -
Metro - The Washington, DC, area's subway system
Purple Line Planning - A WaPo update on Metro's planned Purple Line through Maryland
Traffic - Check out traffic in the area of your potential new home and commuting route
VRE -

Virginia
Alexandria City Real Estate Assessments - Tax records, including sale histories
Arlington County Real Estate Assessments - Tax records, including sale histories
Arlington County's Lot Coverage Rules - Arlington's Lot Coverage Rules for those considering add-ons or tear downs
Fairfax City Real Estate Assessments - Tax records, including sale histories
Fairfax County Real Estate Assessments - Tax records, including sale histories
Falls Church (City) Real Estate Assessments - Tax records, including sale histories

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