Avalance Games offers the third edition of its Panzer Grenadier board game as a free download. There’s a lot of assembly, but its a good way to try it out with minimal cash investment.
WWII
Beer and Pretzels WWII Micro Armor
GHQ has a pdf of its Beer and Pretzels WWII Micro Armor, originally a commerial set, now a set of free wargames rules.