TAG (Tom's Adventure Gaming)

A system that uses d6s.

From the publisher's website:

TAG is a clear, flexible rules matrix that plugs into a variety of settings. The rules are so simple and concise that we've been able to include a complete set of rules with each RuinCrawl, FreeFall, or Cliffhanger scenario, making every one a game in itself - no other books necessary.

From the designer:

TAG stands for Tom's Adventure Gaming. This system is based on a set of rules that I put together during my time in the U.S. Army. It was specifically designed to fit in a cargo pocket-sized notebook to allow for role-playing sessions "out in the field." My original design was heavily influenced by Ken St. Andre and his "keep it simple" principle but, over the years, I have continued to make and refine my own rules matrix.

Today, TAG has become a fairly coherent and smooth-running system, requiring only a smallish, portable book, six-sided dice, and a few friends to get a great RPG session going.