http://www.greatgamesexperiment.com/game/metacraft
I have a friend who has been developing this game for about a year now. Its a 2D sidescroller, where you play a mage. Spells are constructed using a node based editor, where each node is some sort of component of the spell. Mana runs through the connected components like water, activating them and casting the spell.
The screenshots in the link are pretty old at this point, but it gives you the gist of it.
The game is in a constant state of flux, but the goal is to create a simulated world which can be manipulated through the spell system. The simulation includes basic chemical simulation (water+heat = steam stuff), physics simulation and the magic system itself.
The goal is to create emergent gameplay via the massive number of spells which can be created. The game will be single player only (though we have considered how to extend it into multiplayer, its the technology that is the limitation here), and will probably consist of solving physics puzzles.