This is the electronic verision of: Registres et lettres des papes du XIIIe siecle (Rome, 1883) and Registres et lettres des papes du XIVe siecles (Rome, 1899-). These papal letters provide information on many aspects of medieval society. (Updates vary)
http://digital.library.wisc.edu/1711.web/papal-letters
This permalink is a direct, persistent URL to the database that will always work and won't break.