If you want to write Native Client modules (because you want to put C or C++ code in your web app), get the SDK:

Native Client Developer Site

If you're interested in the implementation of Native Client, you're in the right place. We're moving our implementation information to subpages of this one, but some information exists in other websites (such as the nativeclient project, which has a wiki that was used for some documentation previously). The following search field covers all these websites plus groups.