Freeform Email Roleplaying. This is a kind of FFRPG, though the existence of FFRPGs was unknown to the creator when FFERP was invented. The idea is to marry creativity and simplicity. Instead of a hefty rulebook, FFERP has a manifesto:

Freeform Email Roleplaying Manifesto, v 1.0.2
William Van Hecke

Priority 1: Fun.
Priority 2: Advancement of the story.
Priority 3: Enhancement of the game.

Guideline 1: Your character develops as you play. Start with a name; the rest will come to you. Anything goes, but be responsible.
Guideline 2: The world develops as you play. Feel free to add to it as you play. Anything goes, but be responsible.
Guideline 3: There are no dice, stats, or inventories. If it makes sense for your character to be able to do something, assume she can. If it's tricky, indicate that she is trying and the DM will rule. If it makes sense that your character is carrying something, feel free to claim that she is. if it would be more fun to roleplay the situation without it, feel free to say she's left it at home. Anything goes, but be responsible.
Guideline 4: If you know someone creative enough and responsible enough to play, you may invite her. Players may join a group by invitation only.
Guideline 5: If you have any ideas for the improvement of the game, you are invited to offer them.

Yes, it really is that open-ended. Anything more structured than that would be a pain to play over email. Get as involved as you like.