Leeslyvania is in Woodbridge VA, and offers overnight.
Occoquan Park is in Occoquan VA, just north of Woodbridge, and offers overnight.
Pohick Bay Park is in Lorton, VA, and also offers overnight parking.

I have also used Gravelly Point Park near National Airport. The ramps are steep, and the tide REALLY affects the whole experience. Its REALLY shallow on low tide. The place is always JAMMED on the weekend, and parking is a b.itch. It is totally free.

If you have military base access, Bolling Air Force Base has a decent ramp, and marina, in SE Washington DC.

Talk about b@#$h. Try retrieving your boat when the river current is about 2 feet per second. Not to mention floating trees.