SGDK is an open and free development kit for the Sega Megadrive.
It contains a development library (sources included) to make software on Sega Megadrive / Genesis system and a GCC compiler toolchains (Windows system only).
You can find the library doxygen documentation in the 'doc' directory: doc/html/files.html

Website: https://github.com/Stephane-D/SGDK/wiki
You can find basics tutorials about how install and use SGDK on this page: https://github.com/Stephane-D/SGDK/wiki/Tuto-Intro

Unix/Linux users should give a try to the Gendev project from Kubilis: http://code.google.com/p/gendev/
It allows to quickly setup SGDK on a Unix environment.
And now MACOS users also have access to SGDK with Gendev MacOS: https://github.com/SONIC3D/gendev-macos
Thanks to Sonic3D for making it :)

SGDK and affiliated tools are distribued under the LGPL license.