GURPS Nano Lite

Here is a concise summary of GURPS is about.

You have four attributes Strength, Dexterity, Intelligence, Health. Average is 8 to 12 above 20 is godlike

Skills are based on an attribute plus a level modifier generally starting at -2!

You buy attributes and skill levels with points. Disadvantages give you additional points but hinders your character in some way. Advantages can be bought that benefits your chara

Skill are resolved by rolling the Skill level or lower on a 3d6
A roll of 3 or 4 is a critical success
A roll of 17 or 18 is a critical failure.

Barebones Combat

Roll Attack, 3d6 under skill
If successful, defender roll defense, 3d6 under defense
If failed roll damage,
Subtract armor,
Subtract damage from hit points (based on strength),
If you go below zero roll under your health every round until you go unconscious,
If you go below negative health roll once under your health or die,
For every further -5 points of damage roll once under health or die
If you take 5 times your strength you die.

Additional complexity comes in several forms which are optional.

A hex grid to regulate movement and facing.
Special Combat situations like Hit Location
An advanced injury system.

Specific genres; space, horror, fantasy have custom built systems like GURPS Magic. Some are built by manipulating the GURPS Advantages system.

Genres have templates which helps you wade through the myriad choices in the GURPS Core rulebooks. A template is a package of attributes, advantages, disadvantages, and skills. A templates list the cost of the individual items.

You can read more about GURPS here.

GURPS Lite can be gotten for free here

GURPS Ultra-Lite can be gotten for free here.