For those wondering about practical application, here's two quick examples on use with distributed, problem solving and web applications. They're both doing precise models with a focus on deadlock prevention.
I love the pi calculus. It's a good starting point for analyzing distributed systems in a rigorous way. If you're interested in learning more about this kind of thing, check out the Kell calculus[0] and it's related calculi
The pi-calculus is a mathematical formalization of concurrent programs in the same way that lambda calculus is a formalization of computable functions. It is basically a notation that allows you to easily specify concurrent programs, and then rigorously reason about them.
http://www2.cs.siu.edu/~rahimi/papers/2.pdf
http://www.sersc.org/journals/IJGDC/vol8_no5/13.pdf