Fudge dice (also used in FATE-based games and Shadow of Yesterday) are those six-sided die with three different symbols: plus, minus and blank. They’re interested dice to have on hand.
The common use is obvious: between plusses and minuses cancel each other out, you see how much variance you have from your average skill. Ultimately, lots of resolution mechanics are basically answering this very question, and people get it quickly.
The biggest complaint I’ve gotten about this mechanic is that your results towards zero, where the plusses and minuses result in the average result. The actual different may be aesthetic or haptic, but many people do feel something missing from that dice mechanic (as opposed to a 2d6+X style mechanic).
What I also like about Fudge Dice is that their three different sides are useful when you don’t actually care about the raw numbers you roll (as on a normal d6). I’m experimenting with treating Fudge DIce as ways of non-numerically generating 1/3 or 2/3 probabilities. One idea is that plusses generate succeses (just as you roll “hits” in any pool-based systems), while minuses slowly build up other forms of currency.
I’d like to see links to modern-looking and attractive Fudge dice! If I start using them regularly, I could go for some nicer ones.