FireForce! is a set of free wargames rules for 15mm platoon level modern skirmishes.
De Bellis Vietnam is a set of DBA-based quick and dirty rules for wargaming Vietnam.
Here’s a one-page set of free wargames rules for the Chechen Conflict, with figures based on on stands.
Chris Kemp has a mini campaign for his Not Quite Mechanized rules.
Stoelzel’s Models offers a free paper model of a derelict commercial building.