Savage Worlds has bennies, and the greatest thing about them is that I can use them to reward players without giving permanent benefits (such as awarding XP does).
Second, they can be tangible objects (I use poker chips), so they have the feel of a real reward and resource.
As for what they do? A simple reroll on a d20 is best. Rolling poorly is dejecting, which makes using the benny rewarding.
One thing I like to do is find excuses to give pity bennies to a individual player when the dice hate him or her during a session.
I want M&M Hero Points and Complications.
While I wouldn't say I WANT them, I do like the way that M&M does hero points and complications (essentially when a complication or disadvantage comes into play, you get a hero point as compensation IIRC)
Of the various ways I've seen action points used in the past:
- Add 1d6 to a roll Doesn't help a bad roll, but can help you turn a just missed into a success, or help you exceed your normal limits. I like this as a 'pushing yourself beyond the normal bounds' mechanism.
- re-roll Very good at improving bad rolls, doesn't help good roles much. Mathematically improves consistency, BUT in practice I don't like them - it is too much like rewinding time and having another go, it breaks me out of the flow of the game.
- extra action I like this option a lot - it broadens rather than deepens capability
- narrative change I first saw this one in Conan OGL, and really liked it there - stuck in prison you spend a fate point to spot a dropped key or charm a serving wench who luckily is passing to let you out. GM's are encouraged to give over fate points when he screws the party over (ship sinks, all your possessions washed overboard - everyone have a fate point). Obviously that would get right up some peoples noses, but it creates genre-appropriate stories!
Cheers

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.