To me the concept of "Gurevich Abstract State Machine" (formerly known as "evolving algebras") seems very promising to provide a solid fundation to systems behaviours. It could be what relational algebra is for databases: the underlying theoretic fundation.
The most complete implementation, to my knowledge, is the Microsoft ASML language that produces .NET assemblies.
Is anyone really using it in a working context? And, in case, with what degree of success?
I would really like to start investigating more, if others have success stories to share.