Dumbarton Oaks Research Library and Collection supports research and learning internationally in Byzantine, Garden and Landscape, and Pre-Columbian studies. In their site, look for the Resources tab near top, then online collections, digital projects, highlights from the collections, Byzantine studies.
Internet Medieval History Sourcebook
Editor, Paul Halsall, Fordham University. See subject section "Byzantium" along left side. Also the Full Texts Sources section, which has a subsection "Byzantium."