The Nowhere Else and Beyond Game Engine
 The Nowhere Else and Beyond (NEaB) engine is a fully  customizable 2D game engine based on the hugely popular MMORPG  of the same name. The distinctive, and unique, map system allows you  to create a variety of different game situations and the art provided  with the engine is [...]

Deathray Games released one of their games under the creative commons license.
It is a very simplistic, one-player, fantasy hack’n’slash browser game which leaves a lot to do with it.
You can easy adopt it to have it build in other game engines, make it multi player whatever as long you follow the creative commons license.

Game: http://deathraygames.com/play-online/drg/greenhydra/index.html
Download: http://deathraygames.com/play-online/drg/greenhydra/archives/LOGH_v1.zip
Deathray the [...]

Solar Empire is a GPL/public domain browser-based strategy game (BBS) written in PHP. Users control fleets, planets and equipment to conquer the galaxy. Almost limitless variation given by advanced customisation options.

### DOWNLOADS ###
http://www.criminalexistence.com/downloads/Generic-2.9.1.2.zip

Duliga is a PHP written MySQL based multi player online / web game played by Browser. It reverts to real soccer / football results. Users set up a team of real players that competes in a league and various cups aginst other users teams.
Project Admins : duliga_de 
Developers : 1 
Database Environment : SQL-based 
Development [...]

The Dragon Knight game engine is an online game system created by Jamin Seven. It is an RPG-style game in which characters explore the map, fight monsters, gain experience and buy items, among other things. It is written in PHP4, with a MySQL database backend. While it was originally envisioned as a medieval/fantasy type game [...]