The En Framework is a free, open-source set of libraries and utilities used as a framework for the Linden Scripting Language and SLua in Second Life.
Only the SLua modules of En are actively supported. Some of the included LSL modules are unsupported and do not comply with the final versions of GSI recommendations. They are frozen while the SLua verions are being developed and tested.
CLICK HERE for the latest version of En (via GitHub).
You can also browse the repository directly - En is open-source!
En is not compiled; it is imported directly from your computer after downloading it from the repository and unpacking it. Follow the instructions here to set it up with your Second Life viewer.
For an overview of how En works, check out the following pages: