I like the stunting mechanic in Exalted.
The way the rules are written, almost every action that a PC takes that is described by that PC's player in terms other than game mechanics is a stunt.
Most people don't play the game this way, but let's ignore that fact for a moment. I'll get back to it.
So, yeah. Saying, "I attack him with my sword " or "I try to intimidate him (using presence)" isn't a stunt... but if the player puts even the tiniest bit of effort into description (saying, for example, "I run at him, slashing my sword" or "I glare at him, daring him to gainsay me") then the player gets a single stunt die that acts as a bonus on your action.
The effect of this is that people become used to describing their actions in ways that are a bit more interesting... because doing so is more effective.
Now, there are also 2 and 3 die stunts.
A 2-die stunt is just like a 1-die stunt, except that it incorporates a prop or the environment in some way. You pick up a chair to bash someone with? You swing from a tree branch? You move so that the sun is in your opponent's eyes or cause them to stumble over the ridiculous shoes that they are wearing? These are all 2-die stunts.
3-die stunts are a bit harder. They capture those spontaneous moments when everyone around the table just says, "wow."
Stunts also have a resource-recovery mechanic, but that isn't relevant here.
There are a few observations I want to make. Many GMs (including most of the ones I've played with) essentially ignore the stunt guidelines. They ration out only a few stunts a session, and are picky about when they do it. They see stunts as rewards for extraordinary role-playing and description rather than as encouragement for extraordinary role-playing and description. You can run a good Exalted game this way, but you could probably run a better one if you weren't missing out on one of the cooler mechanics.
Really, 1 and 2 die stunts have fairly objective criteria. Players could self-regulate these without the need for a GM referee in most cases. Three-die stunts are, by definition, inter-subjective... but they are also - by their nature - pretty obvious.
I've also played with a player who was very frustrated by the stunt mechanic. He felt like he had to 'try to stunt' and this caused him to stumble over descriptions and such. I've fallen into this trap before, but it was in games where the GMs felt you had to earn your stunts - and I got flustered trying to figure out what would 'count' as a stunt. His problem was a bit different. He struggled with the freedom of stunting. He repeatedly expressed his wish that stunting used some sort of limited resource so that he could spend it and feel like he'd earned the stunt. To me, this misses the point. On the other hand, he was otherwise a good player and would often stunt when he wasn't explicitly trying to do so - I think it was mostly that he sometimes tried too hard.
I was going somewhere with all this...
Something like the Exalted stunt mechanic is pretty easily stolen for other games. I was thinking about that this morning, but I was worrying about people like my friend who tries too hard. As a result, I came up with the idea of Stunt Modes. With this idea, a player can pick one of two stunt modes: Steady or Spontaneous. Someone in Steady Mode can use stunts as normal, but also gets a few dice (or points) of stunt-ness per session that can be applied freely to any roll. Someone in Spontaneous mode doesn't get those flexible points, but gets a bit more oomph out of normal stunts. People who worry that they aren't good at improvisation - or aren't comfortable with the idea of stunting yet - can choose Steady Mode and not get left behind - and work toward increasing their comfort level.