Proas- the original multihulls- are rather rare these days. Which is really quite a shame.
A proa, for the uninitiated, is a laterally asymmetrical multihull: one hull is smaller than the other. Proas, at least in the strict definition, are also double-ended, switching bow for stern in a manoeuvre called a "shunt" when ordinary sailboats would tack. There are, of course, proas that tack in the conventional manner, although these are more commonly called tacking outriggers.
The easiest way to visualize the evolution of a proa is simply to add a single stabilizing outrigger to a dugout canoe, and this is likely how they originated in ancient Pacific cultures. From an engineering standpoint, though, one can easily morph a modern trimaran into a proa (simply remove the redundant windward ama and extend the hull to make it double-ended). Even a modern canting-keel monohull can be morphed into a proa (cant the keel all the way to 90 degrees, and since it's now mostly clear of the water, make it buoyant- and, of course, double-end the hull). In either approach, certain efficiencies are gained in the design: a simpler and lighter structure, a longer waterline (thus a faster boat) for the same weight and build effort, and a few extra degrees of design freedom.
For a more involved description of the proa, the reader is referred to Wikipedia, Proafile, boatdesign.net, Rob Denney and of course the international proa community's Yahoo group, among other resources. My purpose here isn't to rehash that information, but rather to question why the proa concept- despite undeniable engineering merits- holds marginally more than 0% market share.
What are those engineering merits? Speed, for one. R. M. Munroe hit 18 knots in the first 9 m proa prototype he hacked together from second-hand sketches- in 1898. More recently, Russell Brown's 11 m Jzerro- a liveaboard cruiser- has reportedly hit 21 knots, Rob Denney's weight-to-windward Harryproas frequently clock speeds in the teens, and Inigo Wignen's 21.6 m Gaia's Dream is predicted to routinely knock off 240 mile days. The combination of extremely high length/displacement and length/beam ratios with enormous righting moment gives a high power, low drag configuration with no clear "hump speed" to hold it back.
Structural simplicity is another strong point of proas. The nasty torsional loads that cats and tris suffer are not absent, but some of the rig loads are, which can greatly simplify and lighten the structure. And with one fewer hull than a trimaran (and half as much crossbeam), build time and cost are reduced compared to the proa's main competition in the speed department.
From a designer's standpoint, the proa layout opens up a host of new options that would otherwise be pre-determined. Do you put your accommodations to windward, or to leeward, or do you put the social functions into one hull and the private space in the other? How do you distribute the displacement between the two hulls? Since you're using a crazy design anyway, which of several dozen crazy rigs will you use, compared to the handful of basic types seen on more conventional boats?
Most commonly cited proa downsides are easily resolved with some clever engineering. 360-degree or retractable rudders, rigs that can be swung and shifted when the boat changes direction in a shunt, and retractable pod-mounted engines and propellers are complex to design, but relatively straightforward to implement.
That leaves one real and one psychological downside that are keeping proa innovation in the hands of a small group of enthusiasts, rather than production designers. The real downside is length. Simply put, a good proa will almost always be far longer than a mono, cat or tri with comparable accommodations. That's part of how proas get their speed and efficiency; if you build them short and fat, they will perform like pigs. But if you proportion them correctly, they end up more than twice as wide and longer by half than monohulls with similar living space. So either the dock fees triple, or the boat's a sluggish dog- hardly an acceptable outcome for many people. The solution is the one most cruisers seem to prefer anyway: lie at anchor and take the dinghy to shore. It does, however, preclude proa ownership for the majority of casual boaters who are forced to be marina dependent.
Arguably the more important downside, though, is that yachties are traditionalists. After Herreshoff's catamaran Amaryllis roundly trounced the racing monohulls of the New York elite in an 1876 regatta, it (and anything like it) was banned from racing, a mistake that took the better part of a century to rectify. When the simple plywood multihulls of James Wharram and Arthur Piver gave 1960s hippies the opportunity to get out and cruise the world, traditional yacht clubs were so shocked and offended by the radical new boats and their non-traditional crews that multihulls were widely tarred with the "cheap junk" brush, right up until the charter industry realized how much profit could be made by dropping a four-bedroom condo onto a pair of long, slim hulls. Even with all this change, anything new is still greeted with suspicion in many circles. Proas are undoubtedly a much bigger change from the "norm" than cats and tris were, and will face similar if not tougher difficulties in the transition from beach toy to serious cruising boat.