OGRE Miniatures Lite PDF. When I blogged about free versions of some high-profile games, I can't believe I neglected to include this one.
This document has everything you need (except miniatures) to play the classic sci-fi tank combat game. It's a minis game, so you'll also need terrain. If you want to play on a hex mat, just divide the ranges and moves by two. The full version of OGRE Miniatures is more detailed, and it's available as a PDF as well.