Toposolitario has uploaded a terrific free paper model of a steam mecha. It comes in several colors and varieties and would fit in with a steampunk, Weird WWI or Weird WWII theme.
Classic Battletech is a free 24 page set of rules for playing the original game of giant robots on hex maps.
Outland Games presents “Fast Play Mech Rules.” As with a roleplaying game, you begin by rolling up a mech. The entire set of rules occupies just one page.
Mechbrawl is a free expandable card game. In it, you face off against your opponent in a giant, customizable war robot.
Mecha Doom is a set of free wargames rules for 6mm science fiction games. The author writes:
Mecha Doom is a game of apocalyptic warfare where each player controls a cohort of mechs, hellbent on fighting their enemies; either in a bid to gain objectives or to drive them off the field of battle. It is a mech game where the mech remains the focal point of warfare; great walking machines of destruction, standing as tall as a ten storey building – but with enough ‘simplification’ to allow the player to field vast hordes of mechs against each other.