Well RISUS is free and the core rules are around 15 pages and can be used literally for any sort of game, I don't like using rules I don't actually have access to for a game and will not do a pure freeform game since there is not any way to resolve conflicts apart from the GM saying so. A game with no rules is not a game to me.
Okay, I've downloaded it and I found somebody else's RISUS version of the Firefly/Serenity universe.
Give me a few days to become familiar with the rules.
By the way, what sort of things do you want
to do? I presume an erotic adult version of the Firefly setting, but what else, and how far do you want to go?