Jihad is Stan Johansen’s rules for modern insurgencies. There also are printable action cards and counters.
Covering Fire! is a set of free wargames rules on a single page. It is designed for platoon level actions and uses no measuring tape or turn structure.
Patrick Todoroff offers Firefight 2050, a set of rules for 1/72 figures in a near future setting.
MIG Alley Ace are rules for modern aircraft combat from 1945 to the missile age.
Wild In The Streets is a set of free wargames rules for battles between youth subcultures in the streets. Interestingly, the setup is designed so that the game begins when the fight has already started. That keeps players from wasting half the night getting into the action.