Fordham University’s Internet Medieval Sourcebook contains a wide variety of primary source documents from the Middle Ages (that is, 500 to 1500) hosted by Fordham University. From the main page one can find links based on period and category.
The Internet Medieval Sourcebook:
Georgetown University’s The Labyrinth likewise contains a large selection of links to both primary sources and art and art historical materials. Some of the pages have succumbed to “link rot” due to the relative age of the site, but it remains one of the best collections of primary source material available online for free.