National Building Museum
The museum is just a few blocks off the Mall, somewhat hidden behind the Verizon center. For a building that takes up an entire block, it can be deceptively hard to find. Once you have and wander inside (like most other museums in D.C. it does not charge admission) you'll see the bulk of the space is taken up by the open Great Hall. Ringing the Great Hall are three stories of balconies, behind which the rooms (and exhibition spaces) form the perimeter of the building.
The one drawback to the Building Museum tends to be the low ratio of exhibits to building size. There is a permanent exhibit on the history of Washington, D.C. the city - complete with models of the Mall area as it existed at the turn of the century and how planners at that time wanted it to look (which is, with a few exceptions, generally as it appears today). There models and drawings showing the planning process behind many of the big, postcard-star memorials downtown.
The temporary exhibits seem to turn over fairly quickly, and have a wide range of subjects. The first time I went, there was an exhibition on efforts to recreate Shakespeare's Globe theater. The last time, there was a collection of photographs documenting various residence styles across Africa. The drawback tends to be, once again, the surprisingly small size of the exhibition space after to step off the Great Hall. Also, it can be surprisingly hard to find exhibits, since most of the rooms off the balconies are not open to the public, and the exhibits seem to be randomly dispersed with poor signage.
The museum shop, however, makes up for some of the shortcomings. Not only does it have the old maps and prints I'm a sucker for, it crams in more than you would guess at first glance. Obviously the shelves of books on architecture and building design, but also an area of whimsically designed kitchen utensils and implements, as well as toys and other surprises.
Despite the odd use of space, I'm a fan. Especially since I've never once had to fight through crowds, as I have every time I've wandered into the Natural History Museum.