Eli Bendersky's website Some sources compare Google's Native Client with the defunct ActiveX. I think this is just silly. Focusing on its web-directed goals, ActiveX was an attempt to enable writing web applications in C++ and a bunch of other languages like VB. For Windows. For x86.
Native Client is somewhat different. Particularly with PNaCl, it will enable writing a program in C/C++ which will run across multiple OSes and multiple architectures. Now, that is interesting. Imagine the way Javascript is written today. You write a single JS app and it runs inside the browser both on x86 Windows desktops and on ARM Android smartphones. NaCl aims to do the same for C/C++ (and by extension any other language).