AnyDice
An online dice-probability tool with a compact language for pools, keep-high or keep-low rules, exploding dice, rerolls, and custom outcomes, presented as tables, graphs, or CSV.
Why use it
Tabletop, RPG, and loot-system designers can compare success rates, variance, edge cases, and modifier effects before prototyping instead of judging a mechanic from a handful of manual rolls.
Where it fits
Use prototypes and playtests to find core experience problems while they are still cheap to fix.
What to check
Complex programs fail when they exceed the server's roughly five-second computation limit, and a model cannot replace how real players perceive risk; no public source or explicit commercial-use terms are posted, so redistribution of code, site content, or service output requires separate confirmation.