Wrath of Kings is a set of demo rules for a fantasy skirmish system. It has some interesting concepts.
skirmish
18th Century Skirmish Rules
A set of free wargames rules for 18th Century skirmishes, suitable for battles up to 90 figures per side.
State of War ACW Skirmish Rules
Richard Bradley’s State of War ACW Skirmish Rules are available at the Tyneside Wargames Club site.
Bad AR:SE Modern Skirmish Rules
Bad AR:SE is a set of free wargames rules for modern skirmishes.
Late Medieval Skirmish Rules
The Late Medieval Skirmish Rules is a set of free wargames rules for large scale figures on an actual dining room table (with salt shakers, etc.)