I've been using something I've called Hero Points since I started a new campaign in the fall of '02. I hand out cards during play, each of which costs 1 or more hero points to use. A hero point can be used like an action point in that it can add 1d6 to any d20 roll, but the cards are used by my players a lot.
Here are some of the cards that players like:
Hovering at Death's Door: Cost 1. If anything takes you to less than 0 hit points, you may immediately play this card and auto-stabilize at either your current total or -9, whichever is higher.
Sharpen Your Weapons: Cost 2. If you use a P or S weapon, the damage die improves by 1 step for the next minute (i.e. d6 becomes d8, etc.).
Save Me: Cost varies. For every 3 hero points you spend you gain a +1 to any saving throw roll you've made and failed. You may spend as many hero points as you'd like with this card.
The Power of One: Cost 3. If you roll a 1 on d20 for an attack or save, and it would have worked but for the "1 always fails" rule, then you may play this card and ignore the "1 always fails" rule for this roll.
Automatical Critical: Cost 3. If your roll is a threat, you may play this card instead of rolling another d20 for confirmation.
Maximum Critical: Cost 3. If you confirm a critical, you can play this card to obtain maximum damage from that critical.
I have about 30 such cards. They vary in cost from 1 to 5 (with some that are variable). The players can keep them, trade them in for xp (each is worth 500xp, except Hovering at Death's Door which is worth 250xp), or trade them with each other. If a player has more than one character, he has to write the character name on the back of a card so he knows which character it goes with, and he cannot have his characters trade cards (i.e. only trading between players is allowed). A player can play more than one card at a time if he'd like (some cards make awesome combinations). There are some cards that a player can play outside of his turn (like the saving throw cards), including one that a player can play to help another player's character.
Characters get 5 hero points per level until 12th level. At 13th level and beyond they get 9 per level. I also have some feats that cost hero points to activate, but the prerequisite for each is that the character has to be at least 13th level. Characters also get hero points during play. Sometimes a player thinks of something really great, or does something really clever, or role plays really well. In such cases, instead of bonus xp, I'll often give a card to the player, or I'll give some bonus hero points.
Dave