Open Baffle loudspeakers.

i built a pair of the wild burro audio labs using their full range betsy driver. cost under 150 dollars and was a fun project to get into full range, single driver, no crossover speakers. very effecient also and i drive them with a 25 watt fatman hybrid integrated. sound quality is pretty good. but like with most audio there is more to the story. read about open baffle on wickipedia and many other sites on dyi forums. you can get real involved in this with fostex and tannoy drivers which are expensive compared to how i went. also type into "pictures of open baffle speakers" for lots of creative ways some people have built theirs. some people are into open baffle because they do get lots of things right in the midrange and soundstage image. i enjoy mine and may build another pair some day. good luck with this. btw there are companies like hawthorne audio and others that will make some very nice speakers for you if you don't want to build your own.

There are numerous production speakers that use little or no baffle. Here is a partial list:
Vandersteen 1C, 2Ce, 3a
Most electrostatics
Most magnetic ribbon speakers
MBL Radialstradters
Walsh models from Ohm, HHR, Decware, & Audio Physics

...and many more.

I have heard and/or owned a handful of the speakers listed above, and I have always preferred the sound of baffle-less speakers to other designs, especially in the lower price categories. When the budget is bigger, more inert cabinets that pricier speakers have can narrow the gap between baffle and baffle-less speakers considerably, IMHO.

The problem with open baffle speakers is the bass. The rear energy can wrap around and null the front energy. To overcome this, you need either wide baffles or U or H frames. Still, many OB speakers use 12 or 15", high Q drivers. Too high and they get sloppy. (ie. Carver Amazings)

Seems to be more popular among the DIY'ers and kit builders (GR Research) than commercially, not just because a decent box can be a PITA to build. The size and placement (at least 3' from back wall) are factors. Part of the reason for OB resurgence is the availability of subwoofers. Personally, I wouldn't tackle an OB project without DSP because what works for 50Hz might not work for 200Hz.

A popular choice of driver, including coax, is Eminence (particularly Alphas) but AE Speakers' dipole series look good.

As with any kind of dipole, what you lose in vocal "focus" you gain elsewhere.

There is a speaker designer in eastern Iowa that is designing some open-baffle speakers that sound promising while being inexpensive. He is talking about having it in kit form and also in finished trim. The guy's name is Jack Caldwell and he has a company named Holistic Audio Arts. Do a search on the name and it will pull up his info.
Best of luck.