Library of Congress: The James Madison Papers. A welcome addition to the trove of primary documents from our founding fathers that are now available online. Searchable, of course, though since I can’t link directly to a search results page you’ll have to try it out for yourself. (I suggest the searching the keyphrase university of virginia.)
Also of interest: this essay on Madison’s use of code and cipher, which will resonate with readers of Stephenson’s Baroque Cycle. Interestingly, some of his cryptographic correspondents included Edmund Randolph, James Monroe and Thomas Jefferson (this detail is left out of most Jefferson biographies).
Sadly, the images of the papers don’t come with transcriptions. This is a frustrating flaw in an otherwise impressive collection.