CTFs as you need them
Arianne is an engine to develop multiplayer online games like Stendhal
An isometric arcade-style game of skill, puzzles and exploration
A game engine written in the D programming language
Simple data driven web based gamebook engine using javascript.
XNA based game engine completely written in c#
I am working on upgrading the XNA Platform Game Template.
Cross-platform reimplementation of AMOS BASIC (runtime)
The classic intelligent role playing game
Create your own online real-time browser-based MMO in your website
AMOS BASIC reimplementation in Java