The Toposolitario site has a very nice looking downloadable gaming mat for futuristic scenarios.
Warzone Modern battles is a set of free modern Wargaming rules for 1:72 or 20mm Squad/Platoon games.
designed for fairly large-scale skirmishes; up to about 80 men a player. They will do for many periods or settings, so long as troops can be classed into three distinct broad categories of quality. The rules emphasise the importance of sticking together for mutual protection and the value of weapons with longer reach: spearmen can fight in two ranks, pikemen in three. Men in the second rank with javelins can support.
The rules recognise two ways in which figures may be combined, Contingents and Groups. Contingents are real units (e.g. a Contingent of 15 archers from Berwick under their bill-armed vintnar) and are used for organisational and morale purposes in these rules. A good size for a contingent is 32 points. Groups are temporary (sometimes fleeting) arrangements of men occurring in combat situations. Hand-to-hand combat is fought between Groups (which may be as small as one man each) and casualties occur almost at random within the Group
Also, an add-on for leaders and dueling.
Martin Rapier presents a set of free wargames rules for the 1967 Six Day War. In the rules, one base equals one regiment, and a turn equals an entire day.
Border Reiver is a set of miniatures rules for playing medieval and renaissance era games in the border regions between England and Scotland.