ORB LIBRARY: Sources of Primary Documents on the Internet
ORB: Internet Medieval Sourcebook
University of Toronto "Representative Poetry on-line: Version 2.04"
Often just selections but some full texts. Indexed by author and title. But not all texts are accessible to those outside the University; for example, their Riverside 1987 Chaucer is not.
University of Virginia Electronic Text Center
Virginia's Middle English Collection at the Electronic Text Center offers a number of texts listed alphabetically by author: Anonymous (e.g. Owl and Nightingale, Pearl) and works by Dunbar, Gower, Henryson, Langland, Layamon, as well as Chaucer, and excerpts from the Paston Letters and Papers. Searchable as a collection.
University of Michigan's Humanities Text Initiative.
A growing collection of accessible texts online and searchable, including some fifty Middle English Texts.