It should come as no surprise that the British Library has an extensive collection of medieval manuscripts. Much of it has been digitized (as well as manuscripts from other time periods), and you can examine them in fine detail here. You can zoom in so close that you can see the fiber of the paper, every stroke of the quill. It is seriously awesome. The medieval manuscript department even has a blog dedicated to exploring the collection and posting the cool stuff they come across. Check it out!