Rudi Geudens has a page with photos of World War One guns and tanks from the Belgian Royal Museum of the Army and Military History in Brussels. Photos such as these are always useful when painting your wargames miniatures.
Tanks! is a set of free wargames rules designed for playing in your yard.
Dave Ferris offers Panzer III, a set of free wargames rules for WIII tank battles.
Tank Duel is a set of free wargames rules for WWII games where each player commands a single tank.