If you like old school WW2 tile-based wargames, Open General is free on PC/windows (not played it - installed and runs ok though).
Open General is a turn-based strategy game based on Panzer General II and the 5 Star General Series originally published by SSI. It has been coded from scratch and combines the most familiar and popular features from that series, combined with vastly improved computer opponent game play and new features for equipment file, campaign and scenario designers.
Gameplay vid