Everything’s better with friends, and the zombie apocalypse is no exception. After the runaway hit that was last fall’s “Left 4 Dead,” a sequel was an obvious choice, even for developer Valve, a company known for spending years refining existing games rather than releasing slightly updated versions.

The Nintendo Wii was advertised as the video game system anyone could play, with simple, intuitive, motion-sensing controls. But for many Wii owners out there, it has instead become the video game system no one plays: It sits on your shelf, possibly next to your Xbox or PlayStation, and gathers dust until the next time [...]