You'll be able to find more on air forums & wiki, but here's a few to get you started.
257.8 - Towers at civilian airports including RIC
348.6 - Ground control at civilian airports including RIC
255.4 - FAA Flight Service stations
307.2 - RIC approach west
269.4 - Washington Center (this area)
323.0 - Washington Center (SE VA)
349.4 - Air Mobility Command command posts (may be 319.4 also)
372.2 - Pilot-to-Dispatch (USAF version of unicom)
311.0 - Air Combat Command CP's
321.0 - Air Combat Command CP's
If you're in range of the ground stations, you'll also hear the civilian traffic. If you're out of range of the ground stations you'll actually hear military aircraft as they talk to ground. They'll read back frequencies changes ordered by controllers and you can start building your collection.
You may find helicopters using 122.9 or 123.45, and there are also FM freqs in the 30-88 MHz range as well as more AM in the 138-144 and 148-150 ranges.
Traffic's pretty light here since the Air National Guard moved from RIC to Langley, but let us know what you find. The Maryland Forum has maintained an excellent MilAir sticky that's worth following.