I use the campaign system (but not the game rules) from the THW rules, FNG.
http://www.twohourwargames.com/fng.htmlIt's Vietnam based, but would work just as well for any modern patrol type games.
If you're looking for straight combat, tanks, divisional maneuver, etc I am not sure how well it would work for that.
There's also an FNG - Operations unofficial expansion that really takes the campaign system up a few dozen notches. I can't remember where I got that from though, a Yahoo group maybe?