This page is intended to draw attention to lesser known websites, and to offer comments which may help to navigate major websites in ways which are not obviously documented.
[List is work in progress!]
Europeana is a huge collection of material from European libraries; it is almost too big for its own good, and the classic version is more user friendly for academic research. Its search engine requires precise entry (rather than allowing near approximations), but its strength is in the digital collections of libraries such as the Bavarian National Library in Munich, and libraries in eastern Europe. The Munich collection is almost on a par with Gallica for medieval chronicles and similar sources not available elsewhere. It is very good for seventeenth and eighteenth century titles.
Abbreviationes is a very good attempt to tackle a difficult problem, that of reading the many different types of abbreviation used by medieval scribes. It has been developed by Olaf Pluta at Bochum, which I have used for twenty-five years. It started on floppy disks, but is now fully modernised, and available on mobiles. Palaeography doesn’t lend itself to computerisation, and this is probably as good as you can get, particularly if you are someone who uses original documents intermittently.
Series listings are always useful, particularly if they give contents as well as simply the titles when appropriate. For instance, medieval genealogy sites have useful documents on the public records, in particular the official publications. The main medieval entry point is Some Notes on Medieval Genealogy, written and maintained by Chris Phillips. I have found the page on chancery rolls particularly useful.
For original medieval English royal records, the Anglo-American Legal Tradition website contains far more than just legal records, with over 9 million images of rolls and documents in the National Archives. Some of the images are not as good as they should be, which is hardly surprising given the sheer scale of the filming operation.