Electrostatic Speaker designs use a high-voltage electric field to vibrate a thin membrane between two perforated conductive plates called stators. Unless paired with a hybrid design, using dynamic woofers (as MartinLogan speakers do), electrostatics don't need a crossover system the way traditional dynamic loudspeaker systems do.
Electrostatic speakers are best known for their "big" sound, most often referred to as a "wall of sound." They can be hard to drive, and are best matched with a high powered amplifier. Even the largest electrostatic speakers don't excel at bass reproduction, so all are best paired with a good subwoofer.
Other notable companies that make electrostatic speakers are Quad, Apogee, and Essence.
Though similar, planar magnetic speakers (like those from Wisdom and Magnepan) are not the same as electrostatic speakers.
HomeTheaterReview.com's reviews of electrostatic loudspeakers include:
MartinLogan Purity Hybrid Loudspeaker
Quad ESL 55 LoudspeakerMartinLogan CLX Loudspeaker
Apogee Stage Loudspeaker