Stories of adventure were called “romances” in the Middle Ages--a word very nearly divorced from the definition we think of today. Along with romances, there were stories of the lives of saints (called vitae), allegories, and song lyrics--a huge variety when you see them as a whole, but unfortunately not too much of them have been put online. This is a very small selection of an enormous amount of literature.
The Auchinleck Manuscript
The National Library of Scotland has placed this entire manuscript on display online, with commentary, translation, and transcription. There is also a history of the manuscript available and an extensive bibliography, should you be interested in reading further. The website offers search capabilities, a glossary, and a lexicon. If you are new to Middle English or to manuscript studies, I recommend you start with this site.
Wessex Parallel WebTexts: Medieval Lyrics
This site hosts translations, transcriptions, and digitised images of the Harley Lyrics and several other brief pieces of Middle English literature. There is an introduction to grammar, a section on translating Middle English, and notes on technical terms. There is also an index of first lines, for your convenience. If you are interested in medieval song lyrics, this is an excellent site from which to start.
The Bodleian Library: Early Manuscripts
Manuscripts such as “Piers Plowman” (an allegory take about a farmer named Piers who has a dream full of adventures whose morals apply to his time) and Chaucer’s “Canterbury Tales” have been put online here in full, but with no transcription, translation, or commentary. However, if you are interested in studying these pieces, there is an enormous amount of literature about them in other places online or in your local library.
The National Library of Wales: the Mabinogi
The Mabinogion is a collection of mythological tales in Middle Welsh that have a partially Arthurian tinge to them but stand aloof from other Arthurian tales by story elements that are often unfamiliar. These manuscripts are in Middle Welsh without transcript, translation, or commentary, but there are translations online which you can use in conjunction with this project.
The National Library of Wales: A Middle English Miscellany
This manuscript is full of a variety of things, like the Auchinleck Manuscript from the National Library of Scotland, from Arthurian poetry to a treatise on grafting trees. Although accompanied by an introduction there is no transcription or translation readily available online. However, since some of the script is quite legible it might be worth it to challenge yourself by trying to read it straight from the page!