I sat down and played with Allegro.js for a bit this evening, and I absolutely love it!
The first thing I did with it was write a simple "game" where you run around collecting randomly-spawning guitars forever. Super simple stuff. I like the syntax (what I've seen of it so far, anyway), despite it being based off of Allegro 4. And I really like how I can share the game with anyone else without having to hassle with making binaries for every platform. So cool.
I'm not sure what all the library can do (I've just begun looking at it), but I am already thinking of potential games I could write with it.
@Chris Katko: You might like this...
I quickly threw together a simple shim/wrapper for allegro.js that lets you use the library with normal Allegro 5 syntax (think al_map_rgb() instead of makecol(), etc). Check out the attached .zip for details. It even contains a simple "game" where you collect hearts. It's incomplete, but think of it as proof of concept.
I put the shim on GitHub. Feel free to take a look. I added support for basic bitmap tinting and transformations (neither of which allegro.js supports on its own, AFAIK).