XRPG (2nd Ed.)

From the introduction:

The XRPG, an adult role-playing game designed to be played easily over a long period of time via any Instant messaging program. Any number of players can take part, but it is important to balance the number of players with the number of people willing to run and design encounters for them. There is no hard number to the number of players that can play but it is recommended to have at least one extra encounter for every player. Unlike traditional tabletop RPGs, there is no respect to a party system. It’s envisioned as the tale of a single adventurer who has for one reason or another delved into this land of monsters, though you may depending on the circumstance group up to take a particularly difficult quest.