The current version of Lean is Lean 3, and is under active development. The source code and libraries can be found on github.
Each of the following tutorials is available in an online version that runs alongside a Lean in your browser, and as a PDF document.
Caveat: all three are incomplete and under construction.
The previous version of Lean, Lean 2, is still being maintained but not developed. Lean 2 has special support for homotopy type theory and a substantial library. The source code and libraries can be found on github.
You are welcome to join the Lean user forum on Google Groups.