Tool for specifying operational semantics for given formally specified programming language
We have been working on one for the last few years: Lem, a higher-order, typed language with backends for OCaml, Coq, Isabelle/HOL, HOL4, LaTeX and HTML.  Lem has been used internally in the group to formalise various machine models (PowerPC, ARM, and so on), memory models (the C memory model, etc.), programming language semantics (OCaml Light), and so on.