Rogue Space. The Basic Rules are available free for download, and you can print them out in a clever little pocket-sized mini-booklet. There is a "complete" rulebook available for purchase on lulu (a bargain at $7.10), but I haven't received it yet.
Rogue Space is a rules-light RPG system that allows for a great variety of play styles, and great flexibility for running one-offs, or pick-up games. You can boil the rules down into just a couple minutes of explanation, and then every test is just 2d6+relevant attribute against a target difficulty, and then you succeed or fail. It utilizes a more abstract, modern approach to abilities; rather than having an ability score which then has a derived score to apply to the roll, all you have is your "modifier," which is probably just 0 or 1. The result is very similar to the method used in FUDGE or FATE at its core: a roll of 7 (after modifiers) is a success for an average difficulty test, and mitigating circumstances can raise or lower the difficulty by one or more degrees. Opposed tests are similar: both sides roll, the higher number wins. There are options for adding in more boiled-in modifiers for combat, but at its core that's all there is to it. Weapons deal a fixed amount of damage when an attack hits, from light (2 dmg) to extra-heavy (8 dmg). Armor reduces weapon damage by a fixed amount along similar lines.
Personally I think that this system lends itself most readily to a Star Trek style game style; landing on alien worlds, interacting with intelligent species there, occasionally having to get in a phaser fight or rapid flight back to the shuttle. Star Trek, though, in particular, is pretty loose with consistency in what phaser hits are deadly, and which are ultimately grazing blows. A great example of this is the Next Generation episode "Starship Mine" where Geordi LaForge and a Red Shirt both get hit by the same weapon in a similar location. LaForge is fine (though in pain and temporarily disabled), but the other guy is dead. This can pretty easily be explained under Rogue Space mechanics: neither character was armored, and let's say the phaser was a "medium" weapon and dealt 4 damage. If LaForge (a technician) had 4 hp, then the damage would have knocked him out, but he would not be dead (represented by negative hit points). But if the other guy only had 2 or 3 hp (not unreasonable for an NPC), that would have reduced him to below zero hp and he would have perished. Additionally, Rogue Space allows for nonlethal damage. One could also houserule in that, since the other dude was severely drunk, the alcohol would have dealt a point of "nonlethal" damage that made him more susceptible to phaser fire, and it dealt more damage than it more than usual. Most rules modifications are like this: you add one here, subtract one there.
I'm stacking lethal damage on top of nonlethal here, rather than keeping them in separate tracks. My reasoning is essentially that it makes more sense, to me, that if someone was beaten to a bloody pulp with fists and clubs they wouldn't be in great shape. It might not be deadly, but they're not going to be able to do the same things as an uninjured person. And certainly, if that pulped person were to take another hit from something more nasty, they'd probably have a much more high likelihood of going down than someone fresh. Given that initiative in combat is determined by the dice roll plus your current hp, severely injured characters are mechanically "slowed" as well, which supports this assumption about damage stacking.
One can easily get lost in details at this point; if a character is wearing light armor, for instance, and is shot by a gun and the damage is all soaked by the armor, do they still take a point of nonlethal damage from the kinetic force applied to their bodies through the armor? Are there different types of armor to protect against kinetic damage, piercing damage, burning damage, etc.? Using Star Trek as an example again (can you tell I've been watching it recently?), Star Fleet does have "riot gear" when they send in the heavies, but it seems a direct phaser blast still takes someone out, armored or not. Klingon disruptors add another level of complexity, since they seem to ignore armor completely, or at least are powerful enough to pierce through anything less protected than a tank. A disruptor is a small, handheld weapon, which means it probably only qualifies as a "light" or "medium" weapon at the most, but the damage is certainly much more severe than the average phaser setting. Shadowrun and other games have "armor piercing" values for weapons; perhaps certain kinds of energy weapons would ignore a certain number of armor points when figuring damage.
Rogue Space Basic only has three classes: Warrior, Rogue, and Technician. I don't like these names. For a science fiction game, the fantasy roles of Warrior and Rogue don't really "fit" for me; especially because Technician, given the other ones, just seems like a stand in for "Mage." I propose changing the names to Soldier and Scoundrel, for a more archetypal Star Trek or Star Wars sort of feel. Technician is fine, since it hearkens to someone who works in a more scientific or technical station on the ship. Plus, there are psionic rules available optionally as well, so you could easily build a Jedi (for a Star Wars style game) or an empath (for a Star Trek style game). I believe that some people have even developed a 40K addon for this system, which I think just goes to show exactly how flexible it is. For this reason I can't imagine even doing something like Mass Effect wouldn't be unreasonable.
One rule I would change, however, is luck. As it stands, the luck rule (which is optional) dictates that a person can re-roll one dice roll per session. This seems a bit underwhelming. I propose having either a luck "points" system, or else tracking luck and experience with the same points. In order to level, as the rules are written, your character must survive 3 adventures and then can add 1 point to any of the attributes, or to the hit point total. So in other words, one adventure is worth 1 xp, and you need 3 xp to level. Under a unified mechanic, players could "burn" one of their xp (or more) to add a point to their result. It might mean that they don't level with everyone else, but if the other option is dying, it's a small price to pay; it's representative of that character escaping from the deadly situation, but not unscathed. It would also allow the characters to burn a whole level in order to survive from a situation (equal to 3 xp) and, optionally, take a defect to represent their narrow-miss, or a "permanent injury." This makes it a bit grittier than Star Trek usually is, so I don't think many people would miss it if it wasn't there. Regardless I would want luck to play a larger role in a Star Trek (or Star Wars) style game, just because of the nature of the types of heroes in those shows.
Using the experience-burning method of figuring luck could also be applied to more than just dice rolls. Perhaps someone takes lethal damage that would kill their character. They could burn some experience to reduce the damage until they are merely unconscious. This would support a Star Trek style "main cast" character approach, where the characters can survive very unlikely situations relatively unharmed, unless they are Tasha Yar. Although... if a character is going to die, there's not much reason to not burn levels to keep them alive, since if they die those levels are lost anyway. Some sort of constraint might have to be imposed to limit runaway abuse of this system. The up side to character death is that a character can be rolled up in literally seconds, and then could easily be introduced as a new ensign, smuggler, or technical staffer to pick up the slack.
Anyway, this was only intended to be a more or less brief overview of one of the possibilities for adding on to the very smart and simple efficient rules that is Rogue Space, but I got a bit carried away. I'd love to actually run a game of this, either as a one-off or longer, to see how it works, especially how some of these proposals for add-on rules I've made in this work. I have already ordered the extended rules, and I'll have another review of the complete set once I get that in my hands.
One additional note: Apparently this PocketMod version that I am reviewing here is an older version than what is currently available now. In my version, damage is all set values. In the most recent version, they are randomized (light, instead of just 2 set damage, is 2d6, drop lowest, for instance). There is not a date on it, but it says "Copyright 2011 C.R. Brandon." Hit points are the same; random in the present version, set in the previous version. I'm honestly not sure which version I prefer.