I'm wondering about going on a tour of the Pentagon when I'm in Washington next month. I'd be interested in knowing what I have to do to apply, and what you get to see on the tour. Evidently it's free, so if I can go, I'd like to, for sure.
After a quick look at the Pentagon website (http://www.pfpa.mil/visit/access.html), it appears that non US citizens might need to obtain an alien registration card and a social security card, and will need to be escorted during the tour.
However, the language is rather vague and I am not sure what the exact requirements are in order to book a tour. I recommend calling them (their number is at the bottom of the site near the footer) or emailing them and asking for specifics, as the requirements may vary depending on which country you're from.
EDIT: I just tried to make a tour reservation through the Pentagon's website. It is possible for non-US citizens to visit, but like all visitors, they will need to provide 2 forms of ID, which in the case of non-US citizens means a visa/passport stamp and proof of country of origin (passport). One of the forms of ID must contain a photo.
Here is a list of acceptable documents: http://pentagontours.osd.mil/tour-guidelines.jsp
Here is a snippet from the tour reservation email I received, which confirms that non-US citizens can visit:
"Thank you for your interest in a tour of the Headquarters of the Department of Defense, the Pentagon. Please prepare all of your group's personal information (Ex: Last name, First name, Middle name, City, State, Social Security Number, Date of Birth and Passport, Visa, Country of origin - for Non-Citizen) before you request the tour."