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).
Note that some tools requires Java.
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:
https://github.com/kubilus1/gendev
It allows to quickly setup SGDK on a Unix environment.

And now MACOS users also have access to SGDK with Gendev for MacOS from Sonic3D :
https://github.com/SONIC3D/gendev-macos

SGDK and affiliated tools are distribued under the MIT license (see license.txt file)

If you want to donate to support SGDK development:
https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=2SCWVXYDEEBUU

Thanks =)