http://www.nakiti.com/sv4/
It's not finished yet, but this is a freeware program designed to create 2D scrollers (such as Jazz, Sonic, Mario, Megaman, Hocus, Sparkster, Cosmo, Duke Nukum, Aero and however many other games) playable on GBA's or the Visual Boy Advance program. While I haven't actually made anything with it myself, several projects are being worked on, and from what I've seen it seems pretty easy to use. People who don't like complicated design environments may want to check this out, as it seems relatively flexible despite the small range it's designed to work in.