Hordes of the Things.
Used to be free, unfortunately, a new edition has been published, and the editor has remove the free version from its website.
Although some rules are strangely written, it's a very streamlined and elegant system.
The movement phase could be summarized in only one sentence :
spend an activation point to move a unit in any way you like, no corner of the unit shall moves more than its movement rate.
The combat/shooting phase could also be summarized in one sentence too :
each side throw a dice and then apply relevant modifiers, higher score wins (apply result), if you double your opponent's score, it's an epic win (apply result).
There are several kinds of units : dragons, heroes, wizards, knights, light cavalery, spearmen, warbands, hordes, bowmen... even gods. Each type of unit has its cost, its strengths and weaknesses. Some move faster, others are potent fighters, some put their strength in numbers, some excell in the open field, some are best used in ambushes, some may be very strong or weak against a specific kind of opponent, etc.
More on them :
http://ruleonemagazine.com/Iss2/Ru_HOTT_Elements.php
As you can see on the previous link, you can use any mini you want, any scale you want, and build any army you want. Really. That's where the fun begins :
http://www.matakishi.com/15mmrockarmy.htm
There's a catch : the units are pretty much standardized. A unit of Knights is a unit of Knights, period. One unit of Chaos Knights would thus have exactly the same characteristics as one unit of Bretonnian Knights, a unit of Cygnar Storm Lances, a unit of Rohirrim Knights or that unit of mouses mounted on domesticated cats.