SegaGen: The emulator with a secret
Game console emulation is never far from the gray areas of legality. While, generally, emulation is legal, ROM images of games are not.
The Sega Dreamcast has a (somewhat) thriving community of homebrew developers that continue the ill-fated consoles life long after its commercial death. Many legal emulators, media players, and ports of games have been, and still are, being released today.
SegaGen, however, is another beast altogether.
In the early days of the Dreamcast, Sega released a collection of classic Sega Genesis games for Dreamcast, calling it the Sega Smash Pack. Some resourceful person apparently ripped the GD-ROM to his hard disk and began to tinker with it. What he/she found was that the Smash Pack was actually an emulator written by Sega, with a few ROMs included! He/She then reverse engineered Sega's work, and got the emulator to read any ROM placed on the CD with it.
All of the Sega Genesis emulators written for Dreamcast up to that point by individuals were slow and buggy. Suddenly, an emulator appears on a major Dreamcast development site called SegaGen. It ran most games full speed with sound! It didn't take long for someone to discover that SegaGen was just a name that the administrator of the aforementioned site came up with to pretend it was legitimate.
Soon after that, all links were taken down, and Segagen was deemed (correctly) warez. All discussion of the ill-fated emulator was soon banned on the site(s).
Today, the SegaGen emulator is available only on obscure warez sites, usually bundled with ROMs, ready to burn. The SegaGen fiasco was one of the darkest times in the Dreamcast Development scene, and still today, years after the incident, discussion of it is still banned on many large Dreamcast forums.