Loudspeakers where the enclosure takes the form of an open, often flat, barrier. The driver(s) are mounted at, or close to, the centre. The area behind the driver is open.
What's the point?
It's a minimal approach. It's a way to mount speaker drivers so that the sound of the unboxed driver is preserved. The baffle serves simply to secure the driver, and to partially prevent interference from the back wave, to allow good bass. To put a driver into an open baffle is to say "This thing sounds perfect! I don't want to mess it up."
The air behind the driver in an open baffle is *very* loosely enclosed, it does not 'load' the driver into a resonant system the way bass reflex or sealed enclosures do. The driver behave pretty much as if there was no enclosure at all. The open baffle is an excellent design in terms of transient response.
Drawbacks?
Yep. Plenty - otherwise this would be the most common enclosure type. Firstly, to prevent the backwave cancelling out the frontwave, the baffle must be large in comparison to the wavelength of the tone in question, as low tones bend around the baffle more easily then high ones - see diffraction. So these things either have no bass output, or are very physically imposing, well over a metre wide. This large front can be reduced somewhat by folding back the edges of the baffle, in effect making an open-backed box. Alternatively, a fairly narrow baffle can be placed in a room corner, to create an open-topped box. I intend to try this type out, as it seems to me quite a good idea - very easy to make, and not too demanding on living space.
This design also requires robust and / or efficient speaker drivers, as there is no 'springy' mass of enclosed air to protect the driver from over-excursion - see the explanation of VAS in sealed speaker design.
Note: a partial enclosure effect can be achieved by draping thick cloth over the back of the driver. This raises the Q of the system a little.
UPDATE
I know of a couple of European companies that make drivers optimised for open-baffle use.
PHY-HP is a very small operation, selling only a couple of different models of driver. They are sold in matched pairs (the Fs are identical to 1Hz, the resistance is identical to 0.1 ohm etc), built to a high standard and use fancy-pants materials. Fearfully expensive, but they have something of a cult following.
Supravox is a larger firm, and has a wider renge of drivers - including some with field-coil electromagnets (rather than permantent magnets), which is an old-school technology that seems a bit exotic these days. A few of their drivers seem to be very good, at relatively affordable prices. I drool.
Finally, I have recently seen a very simple, yet clever, open baffle speaker. The baffle was made of glass - so it didn't dominate the decor of the room it was in. I gaped in wonderment when I saw it - and "Why didn't I think of that"? ran through my mind. Genius.