Active speakers contain a full speaker system, as well as a crossover network and matching amplifiers. A line input from a preamp or receiver are all that's needed to make active speakers rock and roll. Active speakers need to be plugged into the wall for AC, as they have amps inside. Active speakers are very popular in the pro audio world, led by brands like Genelec and Meyer Sound.
A well designed active speaker can have near-perfect frequency response, as the amps can be EQed to match the drivers. They also require less amp power to create the same amount of output, as there is no crossover to sap power.
The most common active loudspeakers are subwoofers, which usually have built in amplifiers. Soundbars are also active loudspeakers.
Other notable brands with active speakers include NHT, Meridian, Paradigm, Yamaha, and Mackie.
HomeTheaterReview.com's active loudspeaker reviews include:
Meridian DSP5200 loudspeaker
Meridian DSP8000 loudspeaker
Genelec 6010A loudspeaker
Meyer Sound X-10 loudspeaker