DIY audio resources 

Recommended DIY websites
These sites are what I consider to be the best the web has to offer:

Loudspeaker Driver Selection Guide (LDSG)
This site is the DIY bible. Created and regularly updated by Bob Stout, this is an excellent resource and is also the home of the "bass list," an email list with a difference! Share your experience and knowledge or if you are a beginner, ask questions of the more experienced.

Linkwitz Lab
Open baffle dipole speakers, DIY subwoofer
Siegfried Linkwitz is one of the most respected names in audio, and his site is one of the best resources on the web you will find

Elliott Sound Products
DIY electronics audio projects with PCBs and detailed instructions available - including amplifiers, preamps, crossovers, test equipment as well as some very informative articles.

Loudspeaker building
Speakerbuilder.net
DIY loudspeaker designs and articles on loudspeaker construction and design

Argos Audio
Gallery of designs and construction tips on making curved loudspeakers

Loudspeaker design and construction information

Speakerbuilding.com
Gallery of various DIY loudspeaker designs
DIY Audio forums
DIY Audio.com

DIY homepage gallery
Steve's speaker stuff
Very well constructed DIY loudspeakers including open baffle dipoles

Stephen Moore
An excellent site including a modified Phoenix open baffle loudspeaker

Audio Innovation
Dan Marx - DIY audio projects and a good source of information

Kai Audio
DIY projects

Loudspeaker simulation, measurement and testing
True Audio
Loudspeaker design software and articles

Speaker Workshop
Loudspeaker design and testing software
Loudspeaker gallery
KLONE audio
DIY projects & how to create an effective clone of an existing loudspeaker

Critical Q subwoofer
A sealed subwoofer design using the Peerless XLS driver

Loudspeaker types

Transmission Line
Martin King's  MathCAD worksheets
Martin's work on TL simulation is removing the mystery from TL design

TL speakers
Gallery of illustrated TL projects and TL theory

Horn loudspeakers
Horn Speakers
A good resource of horn theory and how to design horn loudspeakers

JHS audio
John Sheerin's DIY horn project

TC ultra-fi engineering
DIY horn projects

Full range driver loudspeakers
Audio Resolution zHorn
Full range driver with rear loaded horn

Loudspeaker kits

Suppliers
This list is by no means exhaustive. If you are looking for a supplier for a particular driver, refer to the LDSG.
Australia
WAR audio
Acoustic Concepts
Mass Technologies
Wes Components
NQLD
GSO Audio

USA
Parts Express
Madisound
Solen
Zalytron
Speaker City
Meniscus
North Creek
E-speakers