Arnold's Finest Hour is a set of free wargames rules for the American Revolution Battle of Saratoga from the prolific Junior General.
Here's a tutorial on modeling birch trees. Like palm trees, these are sadly very much underrepresented among cheap plastic model railroad trees.
Point of No Return is a set of free wargames rules for starship combat with simple vector movement and inertia.
One Monk offers a set of paper miniatures for Star Trek adventures.