Try out the example apps. Its surprisingly easy to get started. On OSX, you can see that when running the Pi example, the Native Client module is using quite a whole load of CPU time (in Chrome, do Window -> Task Manager).
In all that, I learned about Google Chrome’s about:flags. You need to enable Native Client support and restart Chrome to get this all working. There’s a fairly good about:flags resource; clearly lots more things to play with there (and helps me understand what the idea being the ChromeOS/Chromebook is now).