Herzog Zwei was something of a cult hit back in the day. Although not a commercial smash, it had a base of fans who spent countless hours battling for supremacy. I've seen it called the first real-time strategy (RTS) game a few times but never bothered to check the validity of that statement. It doesn't really matter, it was an incredibly fun game that has been overlooked by all the Genesis collections and online retro-gaming services.
Eisfrei is a ROM editor for Herzog Zwei. It has a mostly-friendly interface that makes editing incredibly easy. No messing with hex editors, decoding values, or any of that. If you can click and type then you can hack away to your heart's contentment.
This first release of Eisfrei allows editing of text, unit attributes, order attributes, and tile-based graphics. There are of course many other things that can likely be edited in the game. Whether I go back to find & implement them largely depends on the popularity of this program.