A passive subwoofer is a sub without a power amplifier and possibly without a crossover network. Audiophiles sometimes like to mix and match amplifiers to get better performance from their subs. Today's subwoofer is usually active, meaning that it has an amplifier, crossover and many other features, including sometimes digital EQ, all inside one speaker cabinet.
Many in-wall subwoofers are passive, allowing for the heat-generating amplifier to be placed in a rack and not in a wall.
Over the years many companies have made passive subwoofers, including NHT, PSB, Snell, Totem, THIEL, and Bose.
HomeTheaterReview.com's passive subwoofer reviews include:
Definitive Technology IWSub 10/10 in-wall subwoofer
Totem Tribe subwoofer
PSB CW800E in-wall speakers and CWS10 in-wall subwoofer
Snell IW-Basis300 subwoofer