SLUG

The Simple Laid-Back Universal Game (SLUG) system by Steffan O'Sullivan is about the most "rules lite" rpg system you'll ever run across. If you don't know Steffan O'Sullivan (SOS on line) he is the creator of several role playing systems, the best known of which is FUDGE, and the author of several GURPS supplements. I think it was the increasing complexity of GURPS in the early 90's that lead SOS to move to the much more rules lite systems that he has created.

Although SLUG has been around since 1992 I doubt many roleplayers are aware of it. Why?

Well, probably because it's absolutely free, so nobody can make any money off of it, thus it gets no publicity. Being only available on a single webpage buried deep on O'Sullivan's site probably didn't help it any either.

In a nutshell, SLUG can be written in a few short statements, to whit...

Character Creation --

1. Players describe their characters however they want, using dice, cards, anything; including whatever skills or stats they think are appropriate. The character as written on the character sheet has to be understandable by the GM.
   
2. If the GM approves then that's it, but the GM can amend, modify, or reject any character that the GM doesn't think will work in the game.

Action Resolution --

1. The GM either rolls or calls for the player to roll some dice, how many and what kind are up to the GM or player.
2. The GM adds or subtracts any Modifiers the GM thinks are appropriate maybe based on the character's sheet, maybe based on environmental factors.
3. The higher the roll (with modifiers) is above average the more likely it is the action succeeded. If the GM wants the roll to be low that is fine, too.
4. The GM tells (usually) the player whether and how well their character's action succeeded.
5. Sometimes the GM rolls without telling the players why (often for information gathering or environmental things). Sometimes the roll is public, sometimes it is private.

Character Advancement --

1. During play, if something comes up that isn't on the character's sheet, the player may tell the GM how good (or bad) that character is at that skill. The player should have a good explanation for their character's skill if the GM asks.
   
2. The GM may accept the player's statement, ask for an explanation as to how the character came to have that skill and then accept it, or the GM may reject the player's evaluation of their character's skill level and decide on his own. Maybe some rolling of dice is involved, maybe not, GM decides.
3. Either then, or at the end of the session, the GM may allow a player to add notes to the character sheet detailing new or improved skills and stats.

The End!

...and that's it.

I think you can break it down to 3 simple steps:

1. The GM describes what is and what is happening.
2. The player proposes what his character will do.
3. Loop back to step 1.

Everything else is just fluff.