ONEd6
Intended as a "impromptu" or "pick-up" game, when other plans fall through. Examples below are for a fantasy setting, but could be used for SF, horror, supers, etc.

Universal Mechanic: Only PCs roll dice. Whenever a PC wants to achieve something, or avoid something bad happening to them, the player rolls a GM-specified number of d6. As long as the player doesn't roll any 1s, they succeed.

Standard Checks
No roll - Routine – no significant change of failure.
1d6 - Easy – straightforward, but a measurable chance of failure exists
3d6 - Challenging - chances of success and failure are roughly equal
6d6 - Difficult – only a trained practitioner is likely to succeed
10d6 - Spectacular – only one of the best in the world could succeed
15d6 - Crazy – you want to do what?


PCs
Have a Role: describes how they do things (a 'wizard' uses magic, for example). PCs should describe their actions in a suitable manner for their Role.
Have multiple Traits: PCs start with 5 points of traits. They can have multiple points in a trait (e.g. "Berserk Rage 2"). You can 'spend' a trait after a failed check, so long as the trait seems relevant to the test (GM's call). Each point in a trait lets you re-roll one die that came up with a '1'. However, you have to use all your points in a trait at once (so if you rolled only one '1', and spent a 3 point trait, 2 of those points would be 'lost'). You can only use one trait per check.
Traits reset after each session. You can also reset a trait by good role-playing, or by introducing a cool complication for the PCs. (GM's call)
PCs generally earn 1 extra trait point per session.
All traits are subject to GM approval.


Monsters
Have 4 stats:
Attack – number of dice PC must roll not to be hurt, methods of attack
Defence – number of dice PC must roll to hurt it, defence methods
Hits – how many successful attacks are needed to defeat it
Specials – special attack options in place of normal attack

Example monsters:

Goblin
Attack – 1 (crude weapon)
Defence – 2 (shield, armor scraps, nimble)
Hits – 1
Specials – none

Dragon
Attack – 6 (teeth, claws, tail)
Defence – 8 (scales, size)
Hits – 10
Specials – breathe fire: Attack 4, affects 1d6 targets

Traps are effectively monsters that attack once and then vanish.

Pit Trap
Attack - 3 (fall)
Specials - affects every PC in the front row of the party


Other Challenges
PCs go first, in clockwise order, starting from the GM's chair.
Monsters then go, in descending order of Attack score.


Other Challenges
The GM assigns the difficulty to overcome the task (in terms of dice).
e.g. "Getting past the locked door is a 3d6 check".
PCs then describe a Role-appropriate means of accomplishing the task.
A "Thief" might pick the lock.
A "Barbarian" might smash the door.
A "Wizard" might turn into gas and seep through the crack at the bottom of the door, then open it from the other side.


Damage to PCs
If a PC fails a check against a monster's attack, they suffer 1 point of damage for every '1' they rolled.
The PC must then make a choice: they can collapse unconscious, in which case they are out of this fight but not in danger of dying, or they can try to remain fighting.
If the PC chooses the latter option, they must roll a number of d6 equal to their current damage level. If they succeed, they continue to fight, unimpaired. If they fail (roll any 1s), they are dying and collapse unconscious immediately. They must make the same roll again each round thereafter. If they fail, they die.
Another PC can stabilise a dying PC by spending their action to tend the injured character and making a successful test using the same number of d6 as the dying character's wound level. A stabilised PC need
e.g. Boffit the Fighter rolls three 1s on a defence roll. He had already suffered 1 damage, so this brings his damage to 4. He must roll a 4d6 test to keep fighting. If he fails, he is dying, and must roll a successful 4d6 check each round to stay alive. Anyone trying to stabilise Boffit must make a successful 4d6 check to do so.
All unconscious (but not dead) characters regain consciousness at the end of the fight.


Healing
Rate of healing depends on the tone of the game and the genre being played. "Whatever seems appropriate", in other words. Yes, that's vague, but this is supposed to be a pick-up game anyway :)