#10. Spelljammer Boxed Set
Spelljammer was the first D&D product which broke open the scope of what D&D could be about. It helped a great deal that I discovered Spelljammer at the same time I was taking classes in the history and philosophy of science; realizing this was a toolkit for envisioning Ptolematic and Copernican worlds made real was an eye-opener for me. Spelljammer has permanently influenced my gaming style, and even as recently as a month ago 13th Age characters are finding old spelljammer ships that they can use.
#9. Runequest Cities
When I first discovered Cities it was Midkemia Press and generic; when I later acquired a new print it was now owned by Avalon Hill and rebranded for Runequest 3. Either way this was a fantastic sourcebook for random encounters to bring a city environment to life. I got more out of it then than now, but I have continued to use this book to this day and carry it with me for almost every game, regardless of edition.
#8. Tunnels & Trolls 5th Edition
T&T 5th edition hit me when I was young and relatively new to gaming, around 1983, but I was already growing discouraged with the AD&D methodology. Suddenly here was a set of rules that were easy to learn and play, heavily supported solo gaming (a must have in a remote rural home turf like I had) and also actively mocked AD&D, the first experience I had with any sort of internal elitism in the gaming hobby. It's also the system that established in my mind why treasure is it's own reward and not something you get XP for.
#7. FR: Cities of Gold
This Forgotten Realms supplement for use with the Maztica setting was the first and only effort ever made in gaming to take Pueblo culture and the archaeological studies of the Anasazi and turn it into a setting. It was reskinned for the FR of course, but this was yet another book that hit when I was in college and studying archaeology, so it was a much coveted tome for what it attempted. To date no one has attempted to do this, although there was a Native American inspired sourcebook for B/X D&D (one of the gazetteers) that did an interesting job of condensing its not-American Indians into the world of Mystara.
#6. Planescape and the Manual of the Planes (3E)
This is cheating a bit, but the simple fact is that Planescape permanently altered the way I regarded and approached the planar realms of D&D. The 3rd edition Manual of the Planes did not have the chutzpah of Planescape, but it was a remarkable toolkit for the great wheel cosmology and more, and is a book I continue to use to this day since for reasons unknown Paizo hasn't done much with the planes themselves.
#5. Cyberpunk 2020
The 2nd edition of Cyberpunk 2020 may be a bit dated now and in need of revision (which it's sort of getting thanks to the computer game being developed) but the attitude of the setting, the fantastic writing, the amazing Interlock game system....it remains my favorite hardcore futuristic campaign rules system, and no cyberpunk setting or rules have since lived up to it.
#4. Mayfair's DC Heroes RPG (3rd edition in particular)
I got my first copy of DC Heroes 1st edition at a California game convention as a prize. I was hooked; the cinematic flair of the Mayfair Exponential Game System (megs) was amazing, and the fact that it was DC focused (I've always been a DC fan) was a plus. To this day I wish the game system still lived, and hadn't been sucked away by the people who made Blood of Heroes and then fell into obscurity. In the later semesters of college I also ran what remains to date one of the best campaigns I ever GMed using DC heroes 3rd edition.
#3. GURPS Places of Mystery
GURPS has countless great sourcebooks but this one more than any other has proved fruitful beyond measure and I still have it available as needed today. The book looks at dozens of mysterious real world ruins and sites throughout the world, gives you the run-down on what they are, what makes them interesting, and what their mysteries are. It's great for fantasy and historical gaming, and also fills in a great gap for modern weird adventures. It is 98% system neutral, too.
#2. Call of Cthulhu Keeper's Companion
I can't even imagine running Call of Cthulhu without the Keeper's Companion, the single greatest collection of obscure Cthulhiana for easy access by a keeper that can be imagined. This book has been the source of more CoC games than the core rules ever were for me.
#1. The original 1989 AD&D 2nd edition core books (DMG/PHB/Monster Folio)
Lets face it: for me at least if AD&D 2nd edition hadn't manifested around the time I went to college I might have stopped gaming, focused entirely on my studies and women, gotten a job and who knows, could have been successful like a full decade earlier than I actually was. AD&D 2nd edition hit at the right time for me, and it dominated my gaming obsession for more than a decade. I can't go back to it now, for she is an old, ugly nag now, but boy was she a great ride in the day.