These are the house-rules I in my games...
Karma/Hero Points
Each player starts the game with 1 Hero Point. Thereafter, for every 10 points of experience a character earns, they get 1 point of Karma. Karma points can alter die rolls or generate other effects to the benefit of the players.
Karma Spent & Effects
1 Point: A player may re-roll: to-hit, activation, skill check, Find Weakness, Perception or any other similar roll for determining success or failure. A player spending 1 Karma may Push Above and Beyond as high as they wish without an EGO roll.
2 Points: Players may re-roll any damage/effect roll. They can force any NPC re-roll to-hit dice as described above.
3 Points: Players may re-roll any portion of a damage/effect roll, (i.e., keeping all the 5s and 6's). An NPC can be forced to re-roll for damage/effect.
Variable: Players may modify any to-hit type roll by one point for each point of Karma. NPC rolls may NOT be affected in this way.
Guidelines: A player may spend as much Karma at a time as they wish, but there are limits on how it may be spent. For "to-hit" type rolls, a player can re-roll as often as he wishes until Karma runs out. The same rule applies for damage/effect rolls. Affecting NPCs is handled in the same way. However, one cannot ask the GM to make an opponent re-roll to-hit once damage/effect has been rolled.
Karma may also be spent to modify some of the more nebulous aspects of the game. For example, if the player has Luck, they can elect to spend Karma before the game starts to guarantee good luck. It may not be used to buy-off bad Unluck; it is, after all, a Disadvantage. If a player is doing an activity like searching for somebody, working on a long-term project, and so on, the GM may permit them to spend a point of Karma to ensure success rather than resorting to the dice, especially if it is a plot-relevant activity. As always, the exact circumstances and effects are at the discretion of the GM.
Players may set up a Karma Pool. This is a communal Karma reserve, as it were, from which any player may draw Karma when needed. To build it, a player simply contributes at least one point of their Karma for allocation to the Karma Pool. Only players who have placed points into the pool may use Karma from it. This lets individual members of a team have access to more Karma than their experience would normally allow (one of the benefits of teamwork). If a player should leave a team, they also take their contribution to the group's Karma Pool. Because of this, players need to keep track of how much Karma is personal and how much is in the Pool. In order to use Karma from the Pool, a player must publicly declare that he wants to use Pool points. If at least half of the other players agree, then Karma from the Pool can be used. If not… better luck next time.
Karma does refresh. Whether it does so at the end of each Chapter, or each Session, depends on the scope of the adventure and the will of the GM.