Brian Cameron’s Warfare in Western Europe is a set of free wargames rules for medieval warfare. The scale is 1:50 and the set leans more toward the “beer and pretzels” style of game, as its just three pages long. But that’s a good thing!
Years ago, I played in a play-by-mail game called “Its a Crime.” In that game, you were in charge of a street gang and tried to wrest control of city blocks from the other gang leaders (players). These free wargames rules from the Lincoln Miniature Warfare Society allow you to do the same thing on the miniatures gaming tabletop.
This page describes modern military vehicle camouflage schemes. There are dozens of links and a ton of information. Essential stuff for anyone painting modern military miniatures.
D100 Dungeon is a print-and-play solo game of dungeon crawling. It involves a 53 page book (your laser printer can easily handle it), and uses a series of tables to guide you through an adventure.
Andy Watkins has this page with an amazing list of his ratings on hundreds of historical reference books. It’s worth checking before you take the plunge on a particular volume.