Phantasy Star Online
From Sega Retro
An MMORPG made by Sega for the Dreamcast. This one has a lot of references to past Phantasy Star games. PSO was also released for Gamecube and Xbox (and was one of the only online games for Gamecube). A sequel, Episode 3, was made for Gamecube, but not for Xbox. Lots of times, on the online servers, there would be cameos with Sonic, Tails, Knuckles, and Eggman. There is also a gun based on Eggman's walker in Sonic Adventure 2.