AnyDice: A useful website for calculating dice rolling distributions and odds
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AnyDice
https://anydice.com/
AnyDice is an advanced dice probability calculator, available online. It is created with roleplaying games in mind.
I'd definitely suggest reading the articles and playing around with the tool a bit. Knowing the likelihood of different dice outcomes has been super helpful for me to do things like tailoring loot tables to include item rarity. I thought it might be useful to others as well.