Warstuff is a set of one page rules for playing games with whatever you have in your box. The author writes:
WarStuff is a single-paged ruleset designed to be played with any miniatures or toys you have at hand. The rules are perfect as an introduction to wargaming for kids, and for conventions and pick-up games where players might have models from different lines.
The Perfect Captain has released a campaign system for his Red Actions! rules set. From the Captain:
We have completely outdone ourselves this time! What if I told you this was a campaign game that takes maybe 30 minutes to create from scratch, with limitless scenarios, a different map each and every time, and has counters representing the following armies and their equipment: Reds (All over Russia), Whites (Kolchak, Deniken, Wrangel, Yudenich, Komuch, Kappelites etc), Greens (Mahknovists), Estonians, Latvians, Finns, Lithuanians, Ukrainians, Poles, Armenians, Turks, French, Greeks, Japanese, American, British Empire (Including Canadians, Australians, and Indians), and Freikorps.
Army Men Combat is a set of one page miniature wargames rules. It’s designed for those ubiquitious plastic army men, but in truth could be used for anything.
From Warspawn Games: Celtic Skirmish, a combination miniatures / board game.
Junior General has a simple set of Medieval Castle siege rules. The site also has links to appropriate paper figures, castle models and siege equipment.