Free GM Resource: Easy/Simple Mass Combat Rules

This has been out for a few years (like at least an edition ago), BUT it is largely system-less and can easily be adapted to other versions of D&D or even other RPGs. One thing I see "lacking", assuming it is even needed/desired, is an option for larger-scale, or even mass, combat.

The fine folks over at WotC put out this Unearthed Arcana Battlesystem (pulled from this page). I recommend downloading a copy, giving it a once or twice over, then file away for the 1st/next time you need to figure out how to do some mass combat.