NXEngine is a complete open-source clone/rewrite of the masterpiece jump-and-run platformer Doukutsu Monogatari (also known as Cave Story).
More info and the source for NXEngine here: http://nxengine.sourceforge.net/
More info about Cave Story here: http://www.cavestory.org
NXEngine was written by Caitlin Shaw. Cave Story was written by Daisuke "Pixel" Amaya.
You'll probably need linux (or something linux-like).
# Find a good place to put nacl-nxengine
$ git clone git://github.com/binji/nacl-nxengine
$ cd nacl-nxengine
$ git submodule init
$ git submodule update
$ make
The output is put in $PWD/out. The data needed for the package is in $PWD/out/package.
$ export CHROME_PATH=/path/to/chrome
$ make run-package