Parts Express - These guys carry a ton of stuff, including Vifa, Scan Speak, Audax, Dynaudio, Morel, Hi-Vi, Pioneer, and their own line Dayton Loudspeaker.  They also carry a host of other electrical and home theater equipment and supplies.  They also have quite a bit of clearance and closeout merchandise, for those of you who are cheap, like myself.

Madisound - Madisound carries a wide variety of products, including Vifa, Audax Dynaudio, Focal, Seas, Peerless, Eaton and several others, including the NHT 1259.  They also have clearance merchandise, and one of the better tech boards.

Zalytron - Zalytron sells many of the most popular lines including Audax, Seas, Morel and Focal, as well as some of the more exclusive lines, like Cabasse, Accuton and Raven.  They are also a great source for super-high quality cabinets and system designs.  (Their prices are usually the most reasonable, also.)

Speaker City - This California based company offers products by Focal, Dynaudio, Scan Speak, Raven, Vifa, Versa-Tronics and Pioneer.  Their clearance page usually offers some pretty unbeatable deals.

MCM Electronics - MCM offers tons of electronics parts, including loudspeakers.  The majority of their speaker stock is either OEM or OEM replacement in nature.  They also have quite a few drivers with unique cone materials, including fiberglass, carbon fiber, and aluminum.

WinISD - This is a great program from LinearTeam.  It offers a simple, straight-forward approach to designing a loudspeaker system.  There is a nice user interface with a design "wizard" that walks the designer through the basic steps to designing an enclosure.  One of the nicest features is a huge database of drivers.  You are also allowed to enter your own drivers into the database.  You can see response, phase and power curves.  It also has a signal generator, so you can have it pump out a given frequency through your sound card, or you can have it sweep through the full audible frequency range, so you can test your system once completed.

BlauBox - This program is put out by Blaupunkt.  Its a DOS based program, so its not quite as attractive as some of the other programs, but it does a nice job, none the less.  It's a fairly simple program to use.  It allows designs of sealed, ported and bandpass boxes in different configurations.  A nice feature of this program is that after you have given the program the dimensions of the outside of the enclosure, it generates a drawing detailing the sizes of panels you will need to cut.

ADVspeaker - This is my own program.  I started designing it after using BlauBox and not liking the interface too well.  I had seen several other programs available that cost over $100, and I decided I could make something that worked atleast as well.  It started out as an Excel spreadsheet, then evolved into a pretty simple windows program.  After seeing WinISD, I decided to spice ADVspeaker up a bit.  As such, it is not completely operational yet.  You can plot the response of the speaker, and it will calculate the proper volumes and tuning frequencies, as well as crossover calculations, but the database isn't up yet.  Someday it will also include crossover response plots and system report generation.

PE Box - This is a program put out by Parts Express that uses a database that references their part numbers.  It's convenient if you just want to know what kind of response to expect from a part carried by Parts Express.  I have found that the database is somewhat incomplete, and you are not allowed to enter your own drivers.  Also, the only thing the program plots is -3dB cutoff vs. box size.  It's an odd graph, and really not as useful as an actual frequency response.

Speaker Workshop - This is a great program for anyone interested in testing their speakers.  It allows measurements to be taken for driver's electrical and mechanical properties, frequency response, and several other parameters.  It also includes a crossover simulation tool that includes optimization.  The program requires a full duplex sound card.  For best results, you'll want to build the Wallin jig (or something comparable).  I haven't had great luck wiht this program due to a junky sound card, but lots of others have had very good results.

UniBox - This may be the definitive box design program.  It's a little more difficult to use than some others, but it definitely is the most thorough and accurate.  It allows specifying box losses, stuffing amounts and other parameters that most other enclosure modelling programs don't include.  It also allows display and comparison of closed, vented and passive radiator systems, with lots of different graphs (step response, cone excursion, impedence, etc.)  Requires Microsoft Excel 2000.

Here are some links to manufacturers of loudspeakers, amps, and other speaker-building supplies.

Accuton - Accuton is famous for their ceramic drivers.  They are incredibly stiff, and are supposed to be fairly easy to work with.  They also offer a pure diamod tweeter.  Prices are very high.
Audax - Audax is a French company that produces a wide variety of drivers.  Cone materials range from paper to their proprietary Aerogel.  Their tweeters are highly regarded.  Audax has recently introduced a line of polymer frame drivers that offer high tech cone materials, in a very affordable package.
Cabasse - Cabasse is another French manufacturer.  They are on the upper end of the hi-fi spectrum.  Very expensive products, but also very innovative.  Particularly impressive is their coaxial 3-way transducer.
Dayton Loudspeaker - Dayton Loudspeaker is the house brand of Parts Express.  They offer several different model lines, including home audio, car and professional.  They offer several tweeters, and are great for budget systems.  Their heavy duty series of woofers offer great performance for the price.  Dayton also offers the Titanic subwoofers (10" and 12" models) and the 12" DVC and 15" DVC (which are *very* similar to the Shiva and Tempest).  The Titanics and DVCs are among the top contenders in home subwoofers.
Dynaudio - Dynaudio has recently gotten out of the DIY market.  Finding their drivers now could be challenging.  Dynaudio drivers use huge (3" and 4") voice coils with hexagonal winding.
Eton - Eton is a German company that produces high end products.  These components are a little harder to come by, but are supposed to be really good quality.
Focal - Focal is yet another French company.  They produce a large variety of drivers, most of which are toward the high end of the spectrum.  There are a ton of designs out there that use these drivers.  They are also a little more difficult to find.  Zalytron carries Focal, and offers several kits using their drivers.
Hi-Vi Research - Hi-Vi produces some very nice drive units using rather unique construction.  Many of their drivers feature Kevlar impregnated cones, and all have impressive baskets and motor structures.  They also offer a few good quality ribbon tweeters.
Morel - Is a USA company that produces rather unique drivers.  Their woofers also use huge voice coils.  Their tweeters are considered to be some of the best available.  They also offer a number of small tweeters and midranges that use neodium magnets, making the entire assembly very compact.
NHT - NHT (Now Hear This) is primarily a retail company, but they do produce one of the more popular subwoofers - the NHT1259.  For quite some time it was considered the best sub available, but with the introduction of the Titanic, Shiva, Tempest and Dayton 12 and 15 DVC subs, it has lost the spotlight.  It is still considered one of the great sealed box subs available.
Peerless - Peerless is a Danish company that produces great products.  Their new HDS line is receiving particularly good reviews.  Peerless drivers represent a great value, especially for the quality they come with.
PHL - Makes very high quality professional drivers.
Pioneer - Pioneer produces a large range of speakers for home and automotive use.  They also offer a line of high quality, cast frame drivers.
Raven - Raven is popular for their ribbon tweeters, which are considered to be some of the highest quality ribbons available.  Very expensive products.
Scan Speak - Scan Speak produces some of the finest drivers available to the DIY community.  They are rather expensive, but are capable of great performance.  Most of their drivers use very stiff cones, which can make them a little more difficult to work with unless good test and measurement equipment is available.  Their tweeters may be some of the finest available.
Seas - Seas is a Norwegian company that produces some exceptional drivers.  Many of their drivers are rather inexpensive, but they offer some very high end products as well.  The Excel drivers have received quite a bit of attention.
Adire Audio - Adire sells the Shiva and Tempest subwoofers, as well a few other drivers and amps.  The Shiva (12") and Tempest (15") are considered to be among the best home subwoofers available today.
Skaaning - Was founded by the creator of the Scan Speak and Dynaudio companies, and offers extreme high end products.
Versa Tronics - Versa Tronics offers drivers utilizing high tech materials, for a reasonable price.  Available through Speaker City.
Vifa - Vifa is one of the most recognized names in DIY audio.  They produce great products at a reasonable price.  Their drivers may be some of the most used in speaker building.  Their midbass drivers are popular for their smooth midrange, and solid bass response.  There are several different lines offered by Vifa to suit many different budgets. They also offer some of the most respected tweeters available.  (Incidently, Vifa, Scan Speak and Peerless are all owned by the same company.)
 
 

These are links to other sites that offer good parts, projects or speaker building advice for the DIY-er.

Apex Jr. - Apex Jr. sells mostly surplus items.  They are a great source for sub amps, with models ranging from $25 to $150.  They also carry a few speaker items, as well as transformers, capacitors and various other electronic parts.

SpeakerBuilding.com - Unbelievable projects.  From simple 2-way systems to unbelievable multi-way systems costing thousands of dollars just to build, this site has it all.  Be sure to check out the Andromeda project.  Not only is it incredibly huge and expensive, it is also a very nicely designed project.

Speakerbuilder.net - This is Wayne J's site.  He does a great job of updating it on a very regular basis.  He even has a fairly active discussion board.  The site includes a great Dayton project, and his own loudspeaker testing software, using a Radio Shack DMM and SPL meter.