Doom Turned 21, Play Doom Online!

Doom, Heretic and Hexen - Play the demo version of three remarkable FPS games based on id Software's first generation Doom engine in your browser! The games are ported to Flash by Newgrounds admin mike with Adobe Alchemy, using the original source code of the games.

 
Play the DOOM TRIPLE PACK Here:
http://play.flaswf.tk/PTWBGC/Text/FPS/DoomTriplePack.html

And the open source Freedoom game assets running on PrBoom, compiled to HTML5 with Emscripten, runs in most browser, can be played at
http://kripken.github.io/boon/boon.html
Different from the Doom Triple Pack which uses orignial game code and offical assets, this game uses custom made game resources.

Finally, there is a PC emulator written in Java which can run unmodified Doom.exe in your browser:
http://jpc.sourceforge.net/doom.html (require Java)

Links: http://mochadoom.sourceforge.net/ (Doom Java port)