From Chris Gilders comes No Quarter Fantasy Rules. These free wargames rules are a massive work, with 83 pages in the main rules book alone. There also are a large number of army lists, which is the real strength of the set. Like many fantasy games, figures are defined by a model profile, detailing combat stats, actions, strength, toughness and wounds. This is an amazingly comprehensive work for a free rules set, covering every aspect of fantasy warfare you could want.
Chris Brantley has a page with advice on scratch-building wargames terrain. It's aimed at newbies in general, and DBA players in particular.
Arofan Gregory has posted TIERRA Y LIBERTAD, a set of free fast play wargames rules for the 1910 - 1920 Mexican Revolution.
There is no shortage of free wargames rules for the Colonial period. A good one is the Two Page Skirmish rules from Steve Winter.
Instructions for making a 15mm Aztec Pyramid from David Kuijt. I especially like his solution for making the murals along the sides of the buildings. These are impressive pieces of wargames terrain.