The Middle English texts available online are mainly literary documents from the 14th and 15th centuries, even if one can find other kinds of sources (though relatively scattered). These resources can mainly be found on american websites; at the moment, there is no French site, but a project of the LAMOP (University of Paris I - CNRS) is under construction in a textual statistics perspective. Most of the texts are old editions no longer under copyright (one can find some on Google Books and Archives.org) without, generally speaking, explanatory notes. One exception is the TEAMS project, presented in the Main Sites rubric. Besides, some websites are not freely accessible - I will not present them.
I will firstly present the main sites and secondly the resources themselves, thematically. Indeed, the main sites are often not very well organised and it is not easy to search them. Besides, a thematical organisation allows the consultation of texts on the same thematics but in different locations. But it must be kept in mind that the thematical choices are always subjective and that some texts can be classified in several categories. Conversely, some texts do not find easily a place in a defined category.
Note that the Index of Middle English Verse, with its supplements, is now available online. By definition, it concerns only the poetry. The criterias of research are numerous, but it is not always easy to find a text because they are catalogued by their incipit and not by their current title. Nevertheless, this tool is very useful.
Finally, Luminarium is not website for textual resources, but is dedicated to medieval and modern texts with a selection under control. The work is carefully produced, nicely illustrated and serious, though non-institutional. For each document, there is a notice on the author’s life or the literary genre, links towards the online texts and a bibliography. Founded in 1996, it is regularly updated.