The Ironclaw system uses an interesting variation on dice pools. Multiple dice are rolled, and the result is the highest number showing on any of the dice. With just that, the system would be logically equivalent to a dice pool system where only one success is ever needed.
Where Ironclaw differs, however, is in allowing the dice to be of different types. The interesting effect here is that adding a small die to a pool with bigger dice does not increase the maximum performance possible (the highest number that can be rolled), but does make it less likely that a very low number will be rolled — thereby making easy tasks less likely to be failed.
For example, consider a character with a Dexterity rated as 1d12 (for anyone not familiar with RPG dice notation, it's (number of dice)d(size), so that's one twelve-sided die). The character can perform tasks with Dexterity with difficulty as high as 12 — they'll succeed at such a task 1/12 of the time. On the low end, this character can succeed at a Dexterity-related task with a 4 difficulty on 3/4 of their attempts.
Now, let's give the character a tiny bit of training at, say, lock-picking — enough to give them a lockpicking skill with a rating of 1d4. The character's chance of picking a difficulty 12 lock has not changed. We can say that their training doesn't cover locks that tough, so they have to rely on their innate talent. However, against a simple difficulty 4 lock, the character will now only fail if neither die rolls a 4 or better — giving roughly an 82% chance of success, up 7% from what they had before. A bit more training will raise their lock-picking skill to 1d6, at which point they'll succeed against this lock about 88% of the time.
Ironclaw has a "fumble" rule when all 1's are rolled on the dice for a task — if that happens, then something's been messed up badly. Increasing the size of a single dice doesn't decrease the fumble chance by much; going from, say, 1d10 to 1d12 moves fumbles from 10% of the time to about 8%. However, adding extra dice decreases the chance a good deal; a character going from 1d10 to "1d10+1d4" (that's Ironclaw's notation, even though the dice aren't actually being added) has gone from a 10% fumble chance to 2.5%.
Although Ironclaw doesn't make use of it, such a system also can offer a choice for advancement — expanding the limits of one's ability, or increasing the reliability of it on low-difficulty tasks. That is, someone with, say, a 1d8 skill could have the choice of advancing to either 1d10 or to "1d8+1d4". The former allows the character to do things that they couldn't before, while the latter makes the character's skill more reliable on low-difficulty tasks.
FYI I stole the above text from somewhere else (sorry, no link).
This would be good for an RPG. But it also works great for, say, a wargame (with some modifications). All units on the battlefield might have Attributes: Training, Supply, and Morale. They might have skills like "Bombard" (artillery), "Engineering," "Attack," or "Defense." You could either use the system as is, or flip it around, so you use the LOWEST value -- i.e., an Elite Trained Unit with High Morale is still limited if it has poor supply. Or perhaps the second lowest unit.