Zombie Smashers X is a 2D sidescrolling fighting game for Microsoft Windows from Ska Software. It was released in 2001 as shareware and then re-released in 2006 as freeware. Zombie Smashers has yielded two sequels.
The old website's introduction of ZSX:
Zombie Smashers X has a gripping, fantastic plot in which one or two characters realize(s) that Earth is becoming overrun by zombies and decide(s) to do something about it. The fighting system is basically a tribute to River City Ransom for Nintendo: street brawling with an RPG twist. Each player has stats which can be upped by buying and eating food; special moves are learned by purchasing tattoos. As the game progresses the monsters and bosses become increasingly powerful so it's important to power up. The scenery is pretty diverse and interesting and encompasses one big constant world to explore.
Source: Wikipedia, "Zombie Smashers X", available under the CC-BY-SA License.