It's fast and fun, designed to spur collaboration in character-based emergent stories, and it's built to be hacked.
CORE Micro is the system that lies at the heart of "DayTrippers," but it's been stripped all the way down to its essentials and written with skinning/hacking in mind.
You'll find the current version here:
Published under the Creative Commons Attribution License, you are free to use or modify these rules within your own work, commercial or not, simply by crediting me and the CORE system. I'll even help you promote it! If you decide to publish a hack, feel free to contact me and I'll send you a DOC or ODT version for easier editing.