The original Doom 1 source code has been recompiled (and of course adapted a bit, I guess) using
Alchemy, the compiler which compiles C and C++ into Actionscript:
play it (requires Flash Player 10). There have been rewrites of the Doom engine in ActionScript before, but this is the real original source code running in ActionScript Bytecode, and you can really feel that it is. Great :)