SGDK is a great beginner's introduction to 'C' development on the Megadrive / Genesis; the latest version of which can be found here. I call it a beginner's tool but this needs to be taken with a grain of salt - advanced users can also make very good use of SGDK, but as with all CPUs with similar MIPS and memory limitations best performance will ALWAYS be had with assembly coding. Nevertheless this is a great way to learn Megadrive / Genesis development as the Dev Kit comes with pre-tested functions for most operations and a built-in resource compiler.