I have recently been contacted by members on the development teams of two (completely!) different HTML game related libraries for inclusion on the HTML 5 Development links page.
The first is the Pulse Engine, a traditional HTML5 Canvas based library. Perhaps best of all, it has complete documentation (woot!). I am taking a closer look at this when time permits, and will post something shortly.
The second is Clay.io, which is more of a supporting library. It provides leaderboards, payment processing, social integration and more. They are a commercial library and take a 20% cut of payments processed. It is documented as well, and contains a developer sandbox to test in. I do have one suggestion for the developers… make the features more prominent on your home page. The details were there, but they were below the fold ( on my 1920x1280 screen! ). You will be amazed how many people you will lose because it was immediately obvious what service you provide! If you need to add these features to your game, clay.io is certainly a place to start.
If you have developed and HTML5 related game library, or are a big fan of a library I missed on my list, let me know!