Stars Without Number
Taking a power drill to the system. All these rules are optional and mainly a way to screw around with the system.
AC Overhaul makes AC depend on character level, making armor a HP-booster instead.
Advanced Starship Combat adds advanced rules for starship combat, in which individual systems can be damaged or shut down.
Optional Injuries System introduces flavorful injuries in place of simple unconsciousness or death. This system is still random, so beware!
Item Creation allows you to make and repair objects in the field. This is a risky endeavor, though.
Raising Attributes allows you to invest Skill Points into improving your Attributes, representing training and rigorous workouts.