Lean is an open source theorem prover and programming language being developed at Microsoft Research. Lean aims to bridge the gap between interactive and automated theorem proving, by situating automated tools and methods in a framework that supports user interaction and the construction of fully specified axiomatic proofs.
The Lean project was launched by Leonardo de Moura at Microsoft Research in 2013. It is an open source project, hosted on GitHub.
You can experiment with an online version of Lean that runs in your browser, or try the online tutorials listed under documentation.
Several users maintain the mathematical components library mathlib for Lean.