The House of the Dead
Surprising one, this. The House of the Dead doesn't seem the sort to have a load of spin-offs, but it's actually got a few. The Typing of the Dead, released on PC and Dreamcast (and on PS2 in Japan), is a typing tutor that sees you playing through The House of the Dead 2, only typing words furiously to kill zombies instead of shooting them. It's really good fun and well worth picking up if you can find it.
Above: Zombies and pinball go together like zombies and typing - very well indeed
Then there's The Pinball of the Dead on GBA, which is a solid and fun pinball game, only with zombies to kill with your pinball. Finally, there's Zombie Revenge (Dreamcast) which is a third-person arcade adventure through zombie-infested streets, ending up in the mansion from the original House of the Dead. Memorable for its short-lived but brilliant flamethrower and pulling a pipe from the chest of the first boss before smacking him over the head with it. Blammo!
The House of the Dead isn't the most obvious choice for this list, seeing as it's only had a few additional titles, but all of its spin-offs have been of high quality and at least it didn't resort to a Mario Kart clone. Good work, Zom. Er... you can stop chewing our throats now. Thanks.