HERE. It occurred to me that, while there will be material and different games that I will never be run, I do refer to these books on and off again. I suppose this could compound the problem if trying to cut down in size for one's own gaming collection but, it's an important consideration nonetheless.
I've already mentioned that I play Castles & Crusades. Actually, I've made that admission numerous times on my blog. I have run classic AD&D modules in it and I have taken and used everything from rules, to monsters, and items and spells time and time again. I also have used specific supplements for my C&C game -- most noteworthy being the historical reference supplements that TSR put out for AD&D Second Edition (the green splat books). I have also sourced other material such as the Empire of the Petal Throne and various bits from Role Aids and Judges Guild stuff. Just tons of it really.
I use a lot of this material and take other bits of it in consideration when I do something connected to Swords & Wizardry as well since, for me, it's sort of a no-frills baseline. On the other hand, I don't do anything at all with the DCC RPG. I have it because it's just cool. ;)
Beyond all this stuff, and by extension, Amazing Adventures (currently running as a Kickstarter right now for the new printing and additional books over HERE), is also great resource material for my favorite 'go-to' FRPG. Lots of great stuff and this game has the best rules for Firearms compatible for C&C (and other D&D type games) that I have seen. That said, I doubt I'll have my part of adventurers encounter an Nazi soldier firing an MP-40 at them any time in the near future. Or... maybe I will... I mean if some of those classic TSR adventures has laser guns, why the hell not! ;)
Beyond that, I have other games with 'support material'.
I have a couple of generic d6 RPG books and I use Star Wars d6 by WEG as my support / reference for it. For Call of Cthulhu, I have Chaosium's Basic Core Book as well as a previous edition of Stormbringer and Rolemaster. I have Cortex Core and the original Serenity RPG and the Hackers Guide (for Cortex Plus) along with the Marvel Supers RPG they put out. I have a bunch of Savage Worlds stuff which includes some Beasts and Barbarians stuff to go along with the Fantasy, Horror, and Supers companion.
Frankly far too much stuff the more I look and I think some of this support material could stand to be thinned in the near future too. However, it does justify owning a heck of a lot more than a handful of books. Or maybe I just love excusing my collecting habit. ;)