Coq is described as 'proof assistant, which allows you to write mathematical proofs in a rigorous and formal way, and have them checked for correctness by the computer. It also allows programming with proofs of correctness for the code, and dependent types' and is an app in the Education & Reference category. There are three alternatives to Coq for Linux, Mac, Windows and Online / Web-based. The best alternative is Isabelle, which is both free and Open Source. Other great apps like Coq are F* (Free, Open Source) and Agda (Free, Open Source).
Isabelle is a proof assistant for writing and checking mathematical proofs by computer. It allows mathematical formulas to be expressed in a formal language and provides tools for proving those formulas in a logical calculus.
Isabelle vs Coq opinions
pros, cons and recent comments
Its logic is weaker (simply-typed HOL), but its tooling is much more developed: Prover IDE, integration of automated reasoning systems via Sledgehammer, many add-on tools, big Archive of Formal Proofs
F* is an ML-like functional programming language aimed at program verification. F* can express precise specifications for programs, including functional correctness properties. Programs written in F* can be translated to OCaml or F# for execution.