First up is the original Star Frontiers, available at http://starfrontiersman.com/downloads/remastered.
This is not the D20 version, but the original game first published by TSR back in the mid-1980s. This site includes not only Star Frontiers Alpha Dawn, the basic rules set with various revisions and additions based on years of gameplay, but various supplements and adventures, all in glorious pdf format. The website also publishes an online magazine, the StarFrontiersman, with more valuable information for running Star Frontiers games.
The game mechanics are based on a percentile system. Abilities for most characters range between 25% to 75%, and rolls for most actions are derived from those abilities. For example, shooting is ½ dexterity ability, rounded up (along with various modifiers), and melee is ½ of either a character’s strength or dexterity ability (again, along with various modifiers). Damage reduces a character’s Stamina ability, and if Stamina is reduced to zero, the character is dead. It takes more than a couple hits to kill a character, and advanced technology makes it easier to keep characters alive. Skills modify ability rolls by +10% for each point of skill.
The result is fairly easy system that scales upward very well. Vehicle combat is included in the main rules and is surprisingly easy to run, considering the vintage of the core game. Spaceship combat is included in a separate ruleset called Knighthawks, also available in pdf on the same site. Character creation is very easy, especially when compared to its contemporary, Traveller. Also, unlike Traveller and very much like Dungeons and Dragons, characters get experience from surviving adventures, which they can use to increase their skills or develop new ones, or to increase their ability scores.
The best thing about this game is the wealth of information and supplements available for it. There are seven adventure modules, an expanded game version with more advanced rules, a space combat supplement, and a detailed campaign setting. It’s a lot of bang for no bucks.