The types of manuscripts that have been scanned and uploaded to the internet are various. There are complete manuscripts of highly artistic complexity that have been annotated by scholars and experts, and stubs of parchment without transcription or translation that have been scanned in a low-resolution setting. I have attempted to filter the less professional projects and introduce here only manuscripts that would be helpful to the widest range of study.
A large number of online manuscripts deal with different versions of the Bible, or even of the one book of Psalms, as well as other devotional literature like personalised books of hours. Others are stories of adventure, or moral tales about role models like saints or kings (called "lives" or "vitae"), and essays or sermons on how people ought to live. These tell us a lot about what types of ideals were very strong at that time. A number of musical scores from medieval manuscripts have also been put online, mostly in huge databases rich with information.
The literary subject of most of these manuscripts is philosophical or religious rather than fictional adventures or biography, but it should be recognised that the representation of manuscripts online is not necessarily representative of the sector as a whole: there are many different types of manuscripts, but the best projects online have dealt mainly with religious works. There were stories of adventure (King Arthur, Welsh mythology, Greek poets like Ovid and Vergil) but on the whole they are not well presented online by quantity or quality.
There are, of course, exceptions. However, to group these together would not have given them fair representation since they are not of the same genre. Instead of incorporating them here, I treat them individually, with separate entries as I will for some of the more elaborate or unusual projects.
Please keep this in mind when you read the larger articles I have linked to from this entry! I would not want to present an image that would colour your idea of medieval literature as a whole.