The TomSoft MegaDrive SDK was created by Tommy Xie, lead programmer of Dragon Co., and used by Dragon Co. itself during their Mega Drive era. The package includes a debugger, C++ compiler, MIDI-2-GYM converter and a lot of English documentation on the Mega Drive's chip architecture. In addition, it has an audio player ROM with a lot of reconverted Madonna's songs.
The SDK is available as a free download on Xie's site, but a few additional mirrors have been provided on Pirated Games Central and Mega.NZ