Long story short, I was playing around with random tables and I wanted a way to re-roll a particular die, but exploding die weren't working. So what about imploding dice?
Roll a die. The result is your "marker." Continue rolling until you "implode" (i.e., you hit your marker or more). Until implosion, add the result of each roll to your total.
Though I like the concept of exploding dice, the mechanic skips values. If I were exploding a d6, there's no way to get a result of "6": a roll of 1-5 means I end up with a result of 1-5. But if I roll a 6, then my minimum result is actually 7.  On the re-roll, there's no way to get a "12" because another 6 would explode. So my result sets are based on the number of explosions: 1 roll gives me 1-5; 2 rolls gives me 7-11; 3 rolls give me 13-17, etc.
I mean, I get the idea, and it's all sorts of fun to roll exploding damage. But my OCD cries foul, and it will be heard.
First, it addresses the problem of skipping values. Second, the marker's value suggests (but does not make certain) the final outcome: regardless of the die used, a low marker suggests greater chance of implosion and therefore an overall lower total; a high marker trends toward more re-rolls and therefore a higher value.
With that in mind, GMs can better intuit which die they might want to implode in a given situation, because a smaller die has a higher chance of a low marker, and therefore greater odds of implosion.  Consider, for any given marker, the chance of re-rolling based on the die type used:
OK, again, that was more math than I really wanted to deal with. But I expect people to show their work, so there you are.
Big question: Anyone see this in other games, or have I just invented the best new dice mechanic of 2012? (Roger3, I'm looking at you.)
Or, put another way, there's an 83.3% chance of getting 1-5 and a 16.6% chance of getting a 7 or more. Goose-egg percent chance of getting a 6, which is total crap.
I.e., the chance of rolling the marker or higher.
Yeah, no freak dice, though the percentage for such could easily be calculated: Chance of Implosion = 1 - ((M-1)/S)
Where M equals the marker and S equals the die's number of sides.