I have been working with creating 'trainer' patches for Altered Beast and Golden Axe (level selection, unlimited lives, + other features). I have documented my experiments, collecting bits and pieces into a kind of tutorial on ROM hacking to create trainer patches for games. This tutorial is appended to the existing page on trainers over at SMS-power:http://www.smspower.org/Development/Trainers
My approach might not be the most simple or straight forward way to achieve quick patches like unlimited lives etc. My motivation for hacking ROMs is not only the pleasure of enabling trainer features by patching some isolated bytes (but this IS also a major thrill for me). As a child, I did spend a lot of time wondering about how my SMS games where made, how the console worked, etc. Now, as I'm coming back to all this SEGA 8-bit stuff after 25 years, hacking the games is more like a practice of reworking some of the great mysteries of my childhood. So I'm primarily fuelled by a passion for exploring the inner workings of the games. I'm set on both process and product here.