Reqon - plain text requirements modelling through executable constraints
Based on some discussions from a thread earlier this year I've been playing around with a way of semi-formally defining requirements with a combination of plain text and "executable constraints"
You can think of it as a higher-level, language-agnostic TDD + documentation: combining an informal description with automatically checkable constraints (i.e. individual or collections of tests, scripts, or even a checking of a formal proof assistant)