There are a few spots but very limited to at IAD
. The museum offers great view of side on shots in the afternoon for arrivals on runway 1R. Parking will cost you $12 to get in. It's worth it if you are there all afternoon. 1R is the main runway for all the heavies to land on unless you have a crosswind, they'll otherwise land on runway 30.
The parking garage offers a good view of the airport and best viewed in the morning, I haven't been there yet but parking isnt cheap what I hear. and another area is near Signature Flight Support but alot of photographers have recently been chased out of that area since it's really in the open of airport traffic exiting the terminal, so if you stay there for a long time, chances are that someone will report you to the authorities. You can get some cool takeoff shots of the heavies when using runway 1R. Enjoy!