One Speaker Does the Trick
A Soundbar turns the whole idea of multiple speakers on its head because it seems to be a single speaker. But looks can be deceiving because the single soundbar actually has a number of speakers inside of it. This enables the stereo effect of a pair of main left and right speakers and the center channel, all together in one horizontal unit. Plus virtual surround is added into the mix as well to provide the "surround sound" effect.
An example of this is the Polk SurroundBAR 360. No messy wires, no subwoofer and no special setup. Just put it under the TV, plug it in and use the built-in DVD/CD player to output the picture to the HDTV while it takes care of all the audio needs. The Polk uses a proprietary audio technology to simulate surround from a stereo signal.
And like other companies, Polk makes a passive model- that is to say without built-in amplification or a DVD player- for use with existing audio and DVD system; the SurroundBar in a 42 or 50-inch size.
Another type of Soundbar is Yamaha's Digital Sound Projectors - these use a number of speakers inside the soundbar to distribute the sound by bouncing it off the walls. It creates the surround sound effect by actually having sound traveling around the room (digitally read and controlled from the listener's position via a setup using a microphone). Although a bit costlier than the other type of soundbar, this all-in-one system can be easily moved from one location to the other. As an example, the YSP-3050 compact model includes a switcher for audio/video devices along with varied audio technologies to affect the surround sound effect (and removing the video connections gives you the YSP-900 for even less).
There are plenty of other companies making soundbars, so expect (as in the case of the 2 speaker systems) to look around for the brand and style and functionality of the system that suits you best.