There's a program called Flying Logic that makes it really easy to draw trees and set up mathematical calculations -- sort of a DAG-based spreadsheet sort of thing, with edge weights and logical operators and whatnot. It's marketed mainly for doing cause-and-effect type analyses (using the Theory of Constraints "Thinking Processes") but it can do Bayesian belief nets and other things.

I've played with the demo and it's quite easy to use, but to be honest I didn't use the logical-spreadsheet functionality that much, except to the extent the tutorials show you how to play with confidence values and get the outputs of a fuzzy logic computation.

It's definitely not free OR open source, though.