Squiggle is a special-purpose programming language for probabilistic estimation. Think: "Guesstimate as a programming language." Squiggle is free and open-source.
Read the overview.
The world is made of complex models, but we want answers to simpler questions - what is the most pressing thing to work on? How do we reduce risk? What am I going to have for lunch?
https://www.lesswrong.com/posts/i5BWqSzuLbpTSoTc4/squiggle-an-overview#What_Squiggle_is_Meant_For