I think it would be very useful for people to contribute information about online resources they use for post-classical texts - preferably text files, but also PDFs, scanned books, etc. There are probably hundreds of online medieval Latin books that I make use of right now, so I'll try to be good and post 5 items per day for a while here until we get a good pile going - please chime in!!!
MEDIEVAL AESOP AT THE AESOPUS WIKI: My own personal obsession are Aesop's fables, and I found a lot of great medieval Aesop texts online this summer, which I've listed here - you can see the classical texts, medieval texts, along with Renaissance and later texts listed here, and each page then links to a page at the wiki with either the text or titles that link to the individual texts (linking to specific pages in Google Books is no fun, but it can be done).
BIBLIOTHECA AUGUSTANA: With its chronological indices, this German website makes it easy to find medieval Latin authors by century.
ABERDEEN BESTIARY: For a "wow" moment, this is a website I love to show to people - gorgeous images, accessible text (including English translations) - I love this online book!
WRIGHT'S SELECTION OF LATIN STORIES: This is my favorite medieval Latin anthology, no longer in print - but it is in the public domain and very nicely reproduced here online by Bruce Magee. And no, I don't just love this book because it includes lots of great Aesop (although it does!) - I love it because it really tries to represent the wonderfully wide range of Latin storytelling in the Middle Ages, and reading stories, all kinds of stories, is my favorite Latin reading activity! :-)
I've written a few articles where I needed access to some pretty obscure Medieval texts. But many things have been available through Google Books. Even just in the snippet mode I have been able to get what I needed to both learn the point and create a valid citation.