Emscripten is an LLVM-to-JavaScript compiler. It takes LLVM bitcode - which can be generated from C/C++, using llvm-gcc or clang, or any other language that can be converted into LLVM - and compiles that into JavaScript, which can be run on the web (or anywhere else JavaScript can run).
See http://emscripten.org for project info, getting started, FAQ, etc.
See LICENSE.txt for copyright, redistribution and warranty info.