AARG is my universal system. It's built on a 3d6 mechanic and features over 800 skills and spells on a point-buy system. It's free, and sure to appeal to old school Palladium players.
GENERALA started as a lark and plays the same way. It's a ridiculous, over-the-top, dice-rolling dungeon crawling game. There are also three pre-made dungeons available.
DYNAMO started life as an experiment with different ways to use Fudge dice. What it grew into a is a goofy, narrative-heavy superhero system that plays as a light-hearted take on the nonsense storytelling that was the golden age of superheroes. If your GM is the quick-thinking type it's just about pick-up-and-play.
Another game that started as a joke and became something more. The name means "memorial for a monster", and that's exactly what this game is. Players become kaiju giant monsters and take turns memorializing a fallen monster friend, creating that monster's story as they go. It's really dumb and also so much fun to play.
I was really excited by Chad Wattler's L.U.C.I.D. game - a hack of DREAD for playing in the movie Inception - when he first described it on the Fear the Boot podcast, but at the time he'd only published a one-page summary of the system. I took the idea and fleshed it out with Daniel Bayne's WuShu Open system to give the game action mechanics and the result is a game that's been a pick-up-and-play system my group has used many times. The entire game is improvised and can be played with no preparation.