c-lightning is a a BOLT-compliant Lightning implementation by Blockstream  written in C. Version 1 of c-lightning was released on August 8th, 2015, and has since been under active beta development. c-lightning runs on Linux.
On top of complying with all BOLT standards, c-lightning is developing an architecture for plugins. Plugins can be built to extend the c-lightning command line API.
There are a number of developer tools, libraries and plugins that exist for the c-lightning implementation.