Resident Evil Code Veronica
Although labeled a spin-off due to contractual pressure from Sony, the Sega Dreamcast exclusive Resident Evil Code Veronica was originally designed as the true sequel to Resident Evil 2. It went on to became one of the highest rated Dreamcast games.
Blue Stinger
Blue Stinger released alongside the Dreamcast on 9/9/99, and of all the games that were born on that wonderful day it was the only one that allowed the player to wear a Santa outfit and fire a Gatling gun at a monstrous hermit crab wearing an SUV for a shell.