Horizon Wars is a 2-10mm science fiction combined arms wargame (although some people play it at 15mm, too) designed to be played with whatever minis you have handy.
Over the Horizon adds a wealth of new options, including:
Transforming robots (and other things)
Tactical electronic warfare
Solo and cooperative play rules
Over the Horizon is £15.00 for the PDF, but if you prefer your supplements in the FREE variety, do check out our downloads page for The Maul – a blend of soccer and basketball played by giant robots (and even squishy humans who are mad enough) – and Battle Royale, the ultimate one-on-one mech combat game.