And now, something that isn't Traveller!
Thinking about Trailblazer and d20 a bit, I realized that one of the issues I have with both standard action points and their stronger Trailblazer derivative is that they're completely unexplained in the gameworld; to use the Alexandrian's terminology, they're a dissociative mechanic (caution - loooong series of posts on the other end of this link). No justification is ever given as to why the PCs have them and NPCs don't, except that "Well, they're PCs and they're special." This doesn't quite cut it for me. Even worse is that the resourcing mechanism on the standard implementation is similarly completely dissociated; your pool refills when you gain a level. But the action of gaining a level is typically the culmination of a long, gradual increase in power in the gameworld, which only manifests it in mechanical benefits because the numbers we use in D&D are quantized integers, and it would be a huge pain to implement it otherwise (I have seen a d20 variant that allowed gradual levelling actually; Buy the Numbers. The authors really really liked partial sums). But action points, which are largely uncorrelated with level in their function, need not be assigned in giant chunks at level-up, and it rather strains my suspension of disbelief.
So that's the motivation for this post - I'm displeased with the lack of explanation of action points, though I have no real problem with the mechanics themselves except for their recharge. Thus, the real goal of this post is to attempt to justify action points within the game world, and to provide alternate recharge schema following from those justifications.
The first solution that comes to mind is True20's Conviction, which operates similarly mechanically to action points, and is justified as deep reserves of willpower and energy as a result of a holding strong convictions. It's actually a very similar system to Vampire the Masquerade's Willpower mechanic. In both cases, characters have a nature, and behaving in accord with their natures generates Conviction / Willpower. The fun thing with True20's conviction is that each character's nature includes a virtue and a vice; acting in accord with either restores conviction, but the existence of a vice allows a DM to 'tempt' players with conviction for doing terrible deeds, while the virtue allows DMs to reward them for acting well. This worked pretty well in practice, though because it depends on DM fiat as to whether an action was a sufficiently strong expression of a virtue or vice, it can be tricky. On the other hand, this allows DMs to tune the power level of their game based on their stinginess with conviction, or even to alter the tone of the campaign during play - when you're fighting the good fight against all odds and holding the line against the hordes, conviction might be easy to come by, but when you're wrongly accused, outlawed, stripped of your lands and titles, cursed in the sight of gods and men, then perhaps he backs off the granting of conviction. So that's a bug or a feature, depending on which side of the DM screen you're on.
The next possibility is something like the Glory Points system from Mastering Iron Heroes, where you get 'action points' for sacrifice and good deeds. These are pretty easily justifiable as Good Karma, with spirits and the universe watching out for the PCs who behave well / in accordance with the natural moral order. This also explains why Joe NPC probably doesn't have any; he's unlikely to have held the gates against the orcs to buy time for the women and children to escape. The trouble here is that evil PCs may get shafted; again, depending on who you ask, this may be good or bad.
A third possibility is suggested by the Martial Prowess variant of Glory Points, also from Mastering Iron Heroes, for use in more morally ambiguous campaigns. With Martial Prowess, Glory Points are earned for defeating mighty foes in combat. The karma / spirits justification works here, too, provided that the natural order is violence (which, for something like a Viking campaign, may be true). A word of caution, though - we tried this once at home. The party fighter and the party barbarian got into a competition to see who could earn more; I think we got into the low 30s. We were pretty overpowered, though, and never spent them, because we were hoarding them for the competition (plus we just kind of forgot). Good times.
A final possibility, which is kind of a synthesis of the above 'guiding spirits' and the Ancestor rules from Legend of the Five Rings, would be that each PC has a notable ancestor whose spirit watches out for them and provides them with strength in their hours of greatest need. Here, the actual action points represent the strength of their bond with their ancestor, the strength of the ancestor spirit, and how strongly that ancestor favors them. Retake your family castle? The spirit approves, +1 AP. Spend a month studying his fighting style, slay the descendant of his nemesis, or retrieve his sword? +1 AP. Accidentally drop said sword into a volcano? The spirit is not amused... -1 AP. This also plays beautifully with Trailblazer's action point enhancement rules, which are abilities similar to feats (gained every 3 levels) which effect how you can spend action points and what they can do for you. As you level, the enhancements you take are representative of the knowledge and power the spirit can grant to you as a reflection of its own character; a warrior ancestor might grant Mythic Smite, while a cunning thief ancestor might grant Mythic Luck. Some of the abilities involving passing action points around the party would still be tricky to justify, but possibly workable.
From ancestor spirits, we could also branch out to deities, with action points serving as a measure of your deity's favor, and enhancements serving as a reflection of the boons your god is best at granting to his chosen. Heck, you could even use them to model something like demonic possession / temptation (as with the good Doctor Bones in my last Traveller game - he'd have the Prophecy enhancement, methinks) or the god-in-flesh of a character like Fool Wolf. As long as there's some kind of supernatural entity interested in the character and which acts through means other than direct spellcasting, action points could work, with refresh happening when you particularly please that entity, and the flavors of entity not necessarily the same across characters in the same party.
This approach leaves 'unbound' characters in the lurch, though. What do you do with the grim, atheist warrior who clawed his way up from farmer ancestors, the thief who pays lipservice to many deities but worships none, or the wizard who seeks godhood for himself? I don't have a perfect answer to that yet. Something like conviction would probably be most appropriate for such characters, since the ancestor / god / demon system operates on similar principles in terms of awarding AP.