llvm-gcc (DragonEgg) or
Links to demos, tutorial, FAQ, etc: https://github.com/emscripten-core/emscripten/wiki
Main project page: http://emscripten.org
Emscripten is available under 2 licenses, the MIT license and the University of Illinois/NCSA Open Source License.
Both are permissive open source licenses, with little if any practical difference between them.
The reason for offering both is that (1) the MIT license is well-known, while (2) the University of Illinois/NCSA Open Source License allows Emscripten's code to be integrated upstream into LLVM, which uses that license, should the opportunity arise.
LICENSE for the full content of the licenses.