Formalisms in concurrent and/or distributed programming?
The most widely used formalism is the $\pi$-calculus by Milner, Parrow and Walker. It is an extension of CCS, and comes in many variants, some of which (the asynchronous $\pi$-calculus) attempt to be a formalisation of the actor model. There are now many typing disciplines for $\pi$-calculi, the simplest of which are probably Honda's session types. Such types are slowly diffusing from research papers to research implementations.