You don't need much to run a roleplaying game. You and your players are imaginative & clever. Your characters are where the story comes from. You need a fast simple system that suggests prompts for spontaneous creativity, and stays the hell out of your way.
CORE is a hybrid RPG SRD: it uses traditional concepts & narrativist techniques for character-driven emergent story. Design focus is on simplicity, speed, and improvisation. Everything is on the same scale. All actions use a single Yes/No/And/But mechanic. Results are narratively interpreted. And it's built to be hacked.