If you haven't, it's basically two categories; translations and actual game design changes. While translations are cool, they obviously don't create a new game experience like a level hack does. As for legality, while the ROM itself is illegal, most ROM hacks come as .ips patches that contain no copyrighted code to put onto your ROM (not that you should have one, because they're illegal... *cough*).
One of the most popular games to hack is Super Mario World, mostly because its level editor, Lunar Magic (http://fusoya.eludevisibility.org/lm/index.html) is so easy to use. People have created a lot of brilliant hacks of it, like Mario's Keytastrophe (http://www.smwcentral.net/?p=showhack&id=560), Super Demo World (http://www.smwcentral.net/?p=showhack&id=1), and Mario's Lost World (http://www.smwcentral.net/?p=showhack&id=466).
They've also made hacks of Super Metroid, Yoshi's Island, A Link to the Past, and Chrono Trigger. Among other systems, there's hacks of countless NES games, and even a few Game Boy games (including, yes, Pokemon). I've even seen very rudimentary hacks of New Super Mario Bros.
So yeah... ROM hacking is pretty neat. Discuss it.
One of the most popular games to hack is Super Mario World, mostly because its level editor, Lunar Magic (http://fusoya.eludevisibility.org/lm/index.html) is so easy to use. People have created a lot of brilliant hacks of it, like Mario's Keytastrophe (http://www.smwcentral.net/?p=showhack&id=560), Super Demo World (http://www.smwcentral.net/?p=showhack&id=1), and Mario's Lost World (http://www.smwcentral.net/?p=showhack&id=466).
They've also made hacks of Super Metroid, Yoshi's Island, A Link to the Past, and Chrono Trigger. Among other systems, there's hacks of countless NES games, and even a few Game Boy games (including, yes, Pokemon). I've even seen very rudimentary hacks of New Super Mario Bros.
So yeah... ROM hacking is pretty neat. Discuss it.