The National Archives has had Mathew Brady photos on Flickr for quite some time, though only about half the photos have been uploaded. But I was thrilled to read yesterday that NARA has a) organized the photos into over 40 topical sets and b) geotagged most of the images. Yow! I don’t know the count of images in this collection but I would guess thousands…
The Brady photos are available here on Flickr; this page also shows the sets. Unfortunately Brady’s name is put first in the set, so it’s hard to read the sets for descriptions here. Still, on this page you can see sets for Zouaves, Union generals, railroads, prisoners of war, and camp scenes, and several place names (among other things.)
(There is also a clearly-marked set of images of causalities. This set does have images, sometimes graphic, of wounded and dead people. Please be aware.)
I took a look at the Civil War Entrenchments and Defenses set, which has at this writing 111 photographs. A set of thumbnails was on the front page, with some details available on mouseover. All the images I looked at more closely had geotags and some additional information, though most of them did not have comments.
These new sets make it a lot easier to browse these remarkable photographs. One tip: when looking at individual photos be sure to look at available sizes. All the images I looked at had maximum sizes that were very large — 3000 x 2405, for example — and at that size the level of detail is fascinating and at the same time absolutely chilling.