Winter of 2015, I wrote a series of articles about my favorite GURPS books (you know? the Generic Universal Role-Playing System?), including historical, licensed, genre and original settings. But I didn't actually complete the cycle! What about all those crunch books that AREN'T settings? The books with ready-made (or not so ready-made, scowling at you Vehicles) equipment, characters, monsters, rules and locations; they also deserve some love. Some of them at least. That's a wider net than the term "Splatbook" would normally allow (they're usually about character options like race, class and faction) but it'll do for our purposes. So... what ARE my favorite GURPS Splats?
Nonhuman Races for Science Fiction Roleplaying
Usually covered on four pages each, the alien races included in this seminal GURPS Space book run the gamut from one-offs you might find on a new world (like immovable crystal intelligences) to quirky cultures who might work with humans in the same capacity as Vulcans or Wookies in popular SF franchises, or against them like Predators and Romulans, of course. There really is a bit of everything here, and even the common tropes (the honorable warriors, the hyper-intelligent scientists, the monstrous cannibals, etc.) are given singular biologies, psychologies and cultures that make them stand out. So on the one hand, yes, you can "simulate" the kinds of aliens you need regardless of your campaign style, be it pulpy Star Wars/Barsoom, the clash of empires of Star Trek, or the horror of Geiger or Lovecraft. But your campaigns will also feel unique thanks to these well-drawn races, and provide a lot of surprises for your players. It's not all familiar archetypes either; the book is laced with weirdness too. I've often wanted to run a Star Trek-style campaign where I replaced the well-known (but essentially human with stuff stuck on their faces) races with similar, but more credibly alien, beings from this book. And may yet do so. While this list is alphabetical, Aliens is still probably my favorite book on the list.
Exotic Combat Systems From All Cultures
I'm a fan of martial arts movies, but it's notoriously difficult to get both the mechanics and feeling of them right. I have yet to find a martial arts game/sourcebook that manages to do so. GURPS Martial Arts at least gets the mechanics part right, in a rules plug-in that's actually smoother than it initially looks. The book covers some 50 styles from across history and geography, both realistic and cinematic, and even includes fantasy/SF styles that aren't normally open (or possible) to human beings. Each style is made up of maneuvers (moves) which are detailed in the sourcebook's pages, and which can recombine into other styles, even those you choose to invent. I'll admit it can get a little dry - it's a mechanics splat, after all - but there's enough color describing each style to mostly come off as inspirational, and as with the best of GURPS, I can honestly say I learned something after having read it. There's also a section on martial arts weapons, and a campaign guide you can use not just to introduce martial arts in your setting, but craft a setting that is primarily ABOUT martial arts. (In fact, there's a Martial Arts Adventures companion book with three pretty good adventures of different genres and levels of "cinematic", which I admit I played with Hong Kong Action Theater, not GURPS. However, I did try to 'port as many ideas from GURPSMA to HKAT as I could.)
A Compendium of Marvelous Devices for the Age of Steam
With this equipment splat, the name of the game is FLAVOR. Unlike the dryer text of GURPS' other offerings in the category (Low Tech, High Tech, Ultra Tech), Steam-Tech puts a delightful turn of the century spin on each entry, starting each one with the kind of sales pitch you would expect from old catalogs. And so, the Vulcan Gun is the Scientific Method of Crowd Control, and the Steam Elephant is accompanied by a pithy Kipling poem. The book has everything your Victorian Steampunk explorer needs, whether they be weapons, vehicles (including spacecraft and time machines), analytical engines, automata, drugs & chemicals, armor, tools, communications, and even exotic plants and animals. Very cool stuff, and quite readable even if you're not preparing for a campaign.
Awful Adversaries, Fiendish Foes, and Powerful Punks / The Employment Agency for Metahumans
A bit of a cheat, naming two books, but these two thin volumes complement each other (one villains, the other heroes) and I can't decide which I like more. Where a lot of "original" supers made for games feel rather generic and forgettable, I've found GURPS' ready-made characters often interesting and memorable, and have used several of them in my non-GURPS games. Some say, and they're not half-wrong, that GURPS isn't a great system for four-color heroes, but at a certain power level - let's call it "realistic" or "grounded" - their heroes and villain do work. The books feature some interesting twists on common tropes, and some weirder, more original NPCs as well, and usually provide three levels of power for each character, depending on your tastes. But the mechanics take a back seat to the character's origin and personality anyway, and I've never found it too hard to convert any of them to my Supers system of choice (usually, DC Heroes). Supertemps is also a fun ready-made campaign in the Heroes for Hire mold, an easy, but not boring, way to get heroes into adventures (and the Application Form to become an employee is neat). Super Scum is still probably more useful since your heroes need villains to fight.
Things THEY Don't Want You to Have
I once used this as the basis for a GURPS Black Ops (despite the name, it's really a Men in Black sourcebook) campaign, which basically predicted the Warehouse 13 TV show. Though it does attempt a setting, W23 is really a compendium of weirdness, cryptozoology and Conspiracy relics. It has descriptions (often with extra flavor - FNORD!) of, and mechanics for, everything from the Ark of the Covenant to the Roswell spacecraft to Godzilla. The occult, mad science, Illuminated secrets, they're all here, each one an adventure seed. And there isn't just one Warehouse; you're invited to choose your own location, and different set-ups are discussed depending on your campaign. Who owns the Warehouse? How Illuminated is your setting? As with all of Steve Jackson Games' Illuminati-related game products, Warehouse 23 smacks of dark, satirical humor, and for that alone, is a fun read regardless of whether or not you're playing the game.
Those are my favorites. You may have other ideas (I almost put one or two books in the Horror line, for example). Let us know in the comments!