I run a Swords & Wizardry Complete game - actually, I run two campaigns of S&W. It isn't pure S&W though...
In addition to a ton of houserules of my own, I use some charts from ACKS, magic items from AD&D and adventures written for the DCC RPG (as well as a "Luck" variant inspired by the DCC RPG).
Is it any wonder that mongrelmen are one of my favorite "old school" monsters? :)
So, do you run your games "by the book" or do you use a houserules and rules sourced from other RPGs?