X-plorers is a sci-fi Swords & Wizardry variant. It only uses the d20 and d6, so it is closer to WhiteBox or OD&D feel. From the blurb:
Imagine if the first RPG had been one of interplanetary adventure–rules-light, fast-paced, and inspired by a passion for science fiction.
I bought the limited edition boxed set (still available at the time of this writing for essentially cover price from Noble Knight), but there is also a no-art free PDF available. I love the cover of the box; it is probably one of the major reasons I ended up getting this (though more ideas potentially compatible with traditional D&D are always welcome). There is also a single-volume edition in production, though I don’t think the new cover art is nearly as evocative.
Quick summary: 4 ability scores using 3d6 each (agility, intelligence, physique, presence); 4 character classes (scientist, soldier, scout, technician); several d20-style skills per class with a fixed progression based on level (along with an XP-expensive way to get access to skills from another class). I imagine you could make a simple custom class by just picking any four of the skills.
Technicians have a nifty robotics skill that allows them to reprogram robots to gain minions. The whole little robots subsystem is pretty nice (you can buy software packages to allow robots to do things). Combat is as per basic D&D, though you roll on a d6 critical hits table at zero HP and scientists have a medicine skill which can provide some limited healing. The ship combat system looks like a lot of fun, and uses a series of phases to allow different kinds of characters to contribute in different ways (for example, with successful skill rolls engineers can pump up shields or increase maneuverability).
The Swords & Wizardry single saving throw really shines in X-plorers, and fits a sci-fi game better than a fantasy game (where the atmosphere provided by saves against death rays, dragon breath, and wands is far superior to mundane categories like reflex and fortitude). The way it works in X-plorers is that one is called to make, for example, a physique saving throw. This uses the single save target number (which improves based on level), but the character applies the physique modifier to the roll. This is similar to Third Edition, but requires tracking fewer numbers. Yes, this is also similar to Castles & Crusades, D&D Next, and even some traditional products, but it’s simpler than C&C and preserves the design of saves as dependent primarily on level, unlike D&D Next (from what we have seen so far).
There is an optional psionics system in the Referee’s Guide which could easily be grafted onto any old school fantasy game. The way it is written, any character (like in AD&D) has a very small chance of having a psionic power, and if they have one it is determined randomly from this list: awareness, clairvoyance, telekinesis, telepathy, visions, symbiotic entity. Most of these work like skills with a target number that gets easier as level increases. It also looks like it would be easy to build a psion class by replacing one or two of the default skills in one of the base classes with a psionic skill. I like this system a lot, actually.
Unfortunately, this otherwise good product is marred by some egregious editing and layout mistakes. Four pages are repeated (that’s right, repeated) in the 32 page Referee’s Guide. Some of the transitions don’t respect the actual order. For example, there is a short “actual play” transcript included in the Referee’s Guide which ends: “Now that you know what a role playing game experience is like, let’s learn how to play!” The problem? That passage is on page 23 of the ref booklet, and the only content after it is a short (setting) chapter on traditional space aliens and the psionics chapter (which is labeled as optional rules). I normally wouldn’t spend so much time discussing errors like this that don’t really take away from the game, but come on.