Mongoose has an encyclopedia arcane chaos magic book with a core class that has a free form spell casting system.
EN publishing or whatever it is called now has a supplement for adding chaos magic into the core game (spells, feats and prestige classes). It is available as a pdf and in print.
I have a Wild Mage in the group I'm DMing. He's using the SKR Wild Mage rules. Not as "unpredictable" as the 2ed WM, but falls a little more closer to the original vision of a "Controller of Chaos".
I haven't gotten to run it in game just yet, but it will be soon.