VolunteerMatch – www.volunteermatch.org -- There are hundreds of ways to help out. With VolunteerMatch, it's never been easier to find a rewarding way to give back and make a difference. You can search by location or organization.
Greater DC Cares -- http://www.dc-cares.org/ -- This is Greater Washington's leading coordinator of volunteerism and corporate philanthropy, harnessing the energy, skills, goods and talent of the region for nearly 15 years. Greater DC Cares actively engages the District of Columbia, Maryland and Virginia in unique and rewarding giving experiences that strengthen community organizations and enrich Greater Washington. This “full-service” organization coordinates volunteer opportunities for families and individuals and features special days of service.
Idealist – http://www.idealist.org/volunteer/vol_resources.html -- The Idealist web site lists volunteer opportunities by location as well as those directed to singles, children and teens and families. A great resource that is used by many non-profits in the District of Columbia.