Isabelle is described as 'proof assistant for writing and checking mathematical proofs by computer' and is an app in the Education & Reference category. There are three alternatives to Isabelle for Linux, Mac, Windows and Online / Web-based. The best alternative is F*, which is both free and Open Source. Other great apps like Isabelle are Coq (Free, Open Source) and Agda (Free, Open Source).
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.
Coq is a 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.