I recently began the development of a browser-based game engine from scratch in PHP using MongoDB as a database back-end, in the hopes of building an engine that is cleaner, more maintainable/extensible, more scalable and more secure than many current commercially available engines. I am developing it as a base on top of which I am building my own text-based game (http://www.overloadgame.com), which you can check out if you want to give the engine a little spin.
http://i.imgur.com/3gPXrS0.png
http://i.imgur.com/ccC4Teo.png
I am aware that the game is lacking in many features you would normally expect, but the point was more as a technical demonstration of the underlying engine framework, rather than an actual fully-featured game. In addition, although there are some design aspects that are loosely taken from it, I am trying wherever possible to transition away from the stereotypical MCCodes-derived text-based game atmosphere into something new and more interesting.
If I can get the current (alpha) version of the engine complete and in a vaguely usable state, I'll probably start releasing incremental builds of the codebase for other people to use to build their own games on top of.