A lot of bars, restaurants and cafes have free wi-fi. Starbucks is all over the place and has free wifi, but also all of the major tourist destinations have free wifi as well. Lots of cafes and bars in Sanlitun, Nanluo, Nuren jie, Lucky street, Wangfujing, 798 Art district, etc.
A list of a few that I know of :
Bento & Berries - Kerry Centre Hotel
PaninoTeca at The Village
PaninoTeca at Sanlitun bei
Le Petit gourmand
Comptoir de France