Oxford Dictionary of the Middle Ages [print and electronic] Covers all key aspects of European history, society, and culture from 500 to 1500 A.D., as well as the Byzantine Empire, Islamic dynasties, and Asiatic peoples of the era.
Dictionary of Old English
"The Dictionary of Old English (DOE) defines the vocabulary of the first six centuries (C.E. 600-1150) of the English language, using twenty-first century technology. The DOE complements the Middle English Dictionary (which covers the period C.E. 1100-1500) and the Oxford English Dictionary, the three together providing a full description of the vocabulary of English.
Early English Books Online (EEBO) Contains digital facsimile page images of virtually every work printed in England, Ireland, Scotland, Wales and British North America and works in English printed elsewhere from 1473-1700.
The Early Irish Manuscripts Project, Trinity College Dublin
"focused on four of the Library’s most important early medieval insular Gospel Books": Codex Usserianus Primus, 5th or 7th century; the Book of Mulling, 8th century; the Book of Dimma, 8th century; the Garland of Howth, 8th-9th century.
Digital Scriptorium...is a growing consortium of American libraries and museums committed to free online access to their collections of pre-modern manuscripts.