Coq is a formal proof management system. It provides a formal language to write mathematical definitions, executable algorithms and theorems together with an environment for semi-interactive development of machine-checked proofs. Typical applications include the certification of properties of programming languages (e.g. the CompCert compiler certification project, or the Bedrock verified low-level programming library), the formalization of mathematics (e.g. the full formalization of the Feit-Thompson theorem or homotopy type theory) and teaching.
You can download the current stable version, Coq 8.6, released in December 2016. It features among other things a faster universe checker, asynchronous error processing, Ltac and proof search improvements, generalized intro patterns, a new warning system, patterns in abstractions, a new subterm selection algorithm, and an Ltac profiler!