Subwoofers
Subwoofers have been around for decades, but have only become popular since the advent of Home Theatre. Movie producers use bass to great effect, be it for impact as in a bomb blast or motor crash, or simply to set the mood with an underlying bass note. All A/V receivers have a dedicated bass channel, and while you can get away without a subwoofer it is not advisable.
Subwoofers for stereo applications can be more critical than for movies. In a Home Theatre environment your senses are distracted by the film itself, while when listening to music you are totally focused on what is coming from your speakers. A poor subwoofer tends to make all bass notes sound the same. They may reach down into the lower registers, but there is no definition. If you are listening to acclaimed bass player Charlie Hayden you want to hear all the subtleties in his music, not simply a flow of mono-tone notes.