Airland War is the successor to Steve Lorenz' successful, commercially published Panzer War miniatures wargames rules. The rules simulate modern warfare using microarmor miniatures. You can now get Airland War as a free download.
Will McNally has this set of free wargames rules for the American War of Independence. What makes these rules different is that they come with free software! Once the orders of battle are set up, the rules are run by a computer through clicking on options with a mouse. The rules run on MS-DOS, but can, of course, be run in a DOS box in Windows 98 or XP. If you're like me, and have an old MS-DOS laptop sitting around, this could be a perfect use for it.
I've always thought that there was great potential for using laptops in miniature wargaming. Even better: someone should write a program for managing tabletop games with a PalmPilot.
Matt Fritz has an article on how to keep the paint on plastic miniature wargaming figures.
If you are playing miniature wargames set in the Sudan, you've got to have a redoubt. Steve Winter has a very detailed explaination of how to build one.
Imagine Image offers the freeHex Command Ancients Rules for miniatures wargames. The hex system is supposed to simplify things and speed play. You can play without a hex board if you want.