De Re Military - Your portal to warfare in the Middle Ages - Website of the Society for Medieval History). Founded in 1992, De Re Militari is an academic organization for the study of medieval warfare, broadly construed. The Primary Sources page includes Primary (and secondary) sources on medieval and renaissance military history and warfare. (Currently (2020) It links the old pages from 2011 due to work on the website).
EuroDocs: History of Medieval and Renaissance Europe - Primary Documents - (Contents: 500 AD to 1600, plus a link to Special Collections). This Section is a a sub-set of the larger EuroDocs: Online Sources for European History .
Hanover Historical Texts Collection - Digital versions of historical texts for use in history and humanities courses. Includes a small collection of letters of crusaders.