Fudge is a roleplaying game by Steffan OSullivan that has been freely available on the web since 1993. Fudge provides a simple and intuitive mechanic for resolving situations that arise during a game. Rather than numbers or percentiles (common to many roleplaying game systems), Fudge employs words to describe how well or poorly characters are skilled, how challenging or simple a proposed task is, and how beneficial or adverse are the results. Fudge also provides scads of information about how to customize the rules for your particular needs.
Terra Incognita includes a complete, customized version of the Fudge rules no other books are necessary for play.
You can download the original Fudge rules from Grey Ghost Press or purchase the printed version, Fudge Tenth Anniversary Edition, which includes a plethora of extra goodies.
You can find out more about Fudge from Grey Ghost Press and the author, Steffan OSullivan.