This is project created by Kevin Gadd and here is his description of how it works:

Check out the the demos:
- Try writing C# here: http://jsil.org/try/
- Tetris game: http://hildr.luminance.org/Tetris/Tetris.html
- Lumberjack game: http://hildr.luminance.org/Lumberjack/Lumberjack.html
- RayTracker demo: http://hildr.luminance.org/Raytracer/Raytracer.html
- Mannux game: http://hildr.luminance.org/Mannux/Mannux.html
In addition to a really powerful combination of technologies, here is what I really like about this site:
- Clearly explains what it does
- Allows the user to Try it now (of course that running the code in the browser helps)
- REPL environment on browser
- Very social with direct links into creating GISTs with the code created
- First step to using it is a Git Pull and active encourage of Git Forking
- Little mascot :)
I really want to see if I can integrate this with the O2 Platform since this could be the missing piece that I was missing to create jO2: the port of parts of O2 into Javascript :)