U.S. State Department Country Profiles
Library of Congress Country Studies
Google Earth This free, downloadable map software has more features (layers like 3D buildings, borders, etc.) than its strictly web-based counterpart, Google Maps.
Photosynth - Unlike Google Maps and Google Earth, Photosynth is good for interior, panoramic imagery. Mostly of well known places (e.g., the Sistine Chapel) but these do give you a sense of being there.
Google Maps and Google Earth (again! - aside from the aerial photos and satellite imagery, these offer links to photos taken at many sites)
TinEye Reverse Image Look-Up: This is a new site, so there's still a lot of kinks in it, but you can upload an image to it and it'll find other places on the web where this image is. It's great if you have a picture you want to find out more about.
Note: Architectural information also tends to be regional and the following are just a few examples. You might also look for travel guides or histories of specific cities or regions.