There are currently a wide range of good sets of miniatures game rules available for free on the internet. My particular favourites out there are:
Fast and Dirty FAD from Weasel Fierce's website. This is a set of SF large skirmish rules for platoon to company level actions with no specific background specified and a design influenced by Stargrunt II.
Stargrunt II from Ground Zero Games is an interesting set of SF miniature rules for large ground skirmishes. They are possibly a little fiddly in use because of the different types of dice used and the mechanics for resolving fire fights needing multiple die rolls.
Full Thrust is a set of starship combat rules from Ground Zero Games, who produce an extensive range of human and alien nations' starships to use with these or any other sets of suitable rules. Full Thrust does give a fairly quick and enjoyable game and can be used with any set of starship models that you want.
GZG also have Dirtside which is a set of larger scale ground action rules intended for use with 6mm or similar small scale miniatures.
The fantasy miniatures rules Hordes of the Things have now been made available for personal use till the next edition appears at some future time from the Wargames Research Group.There are quite a few other sets of miniatures rules avaialbe for free from the WRG website. The old version of the DBA rules is also there and that provides a fun game though it is somewhat of a chess with miniatures game.
There are also a large number of sets of rules at Freewargames Rules, but I haven't tried any so can't really say more than point people at them.
I'll post some links for the paid for rules that I favour in a separate entry on the blog soon.