Ask HN: Uncertainty / Monte Carlo Analysis Tools?

I recall once seeing here a web-based tool that allowed for some really simple uncertainty modeling. It was visual and node based: you could drag parameters around as inputs to functions. Each parameter could have a distribution type set to it (i.e. uniform, normal, etc) and the simulation would evaluate the function by sampling the parameters per their distribution type. You could then chain together these functions to end up with a result.

Does anyone know of a similar tool?