Armored Ink Blog has an article about making cheap trees from nails.
Be careful. A wargaming pal of mine did this and at one point, lost his balance, tried to catch himself and put his hand right down on one of his trees. Drove the nail right through his palm.
Ouch.
Forge of War is a set of free science fiction skirmish rules from Craig Cartmell. He writes that they were born of frustration with the Warhammer 40K rules from Games Workshop.
Gerard De Gre and Charles Sweet were pioneering wargamers who produced a number of rules sets, including these Rules For Napoleonic Wargames, which is played on a 4 inch grid.