St. Thomas Aquinas. This website will eventually have all of Aquinas works, and in the meantime has the best search functionality available, and it both displays and searches both Latin and English. If you need a work in Latin that is not at this site, go here. If you need a work in English or Latin-and-English that is not at this site, go here.
St. Albert. You can search Albert’s works online or download .pdf files at this site.
Lombard’s Sentences. Not one of the greats, but worth inclusion on this list. Latin text only.