Audio Links

Software Tools:

Speaker Workshop
Horn Response THE horn simulator. Works great for traditional front-loaded horns.
Unibox Excel sheet for doing T/S box design.
Circuit Maker Helpful little Spice app for designing active filters

Speakers:

Linkwitz Lab Siegfried Linkwitz's page - details on measurement mic construction and active filters.
Art Ludwig's Homepage Information on sound, room acoustics, and speakers.
Timo Christ's Homepage Horn stuff, plus amps, etc. Home of the horn foldings page.
The LDSG Bob Stout's DIY Loudspeaker selection guide.
James Melhuish's Horn Pages Horn info and a fullrange driver forum
Adire Audio Their XBL2 technology looks interesting. I heard a blind A/B demo of it in a midrange speaker driven very lightly, and it was easily audible.
Madisound A great source for speakers. Very good service, in my experience.

Other fun stuff:

The Scientist and Engineer's Guide to Digital Signal Processing An easy to read introduction to DSP.
Steve Bench's Webpage DIY tube projects
John Broskie's Tubecad page Tons of ideas and information on tube projects
Helmholtz Resonance
Helmholtz Resonators
Flow in ducts
Turbulent Flow
Viscosity
Bicycle Aerodynamics
Eric's World of Physics Tons of information. Comes in handy sometimes. Other times it's a bit cryptic.
DJ Aerotec
Vacuum Bagging Wings
Cheap Vacuum Pump
Make Your Own Vacuum Bagging Equipment
Vacuum System Project
A Vacuum Gauge
Foam Cutter Links
Backyard Metal Casting
Material Properties - this is an awesome resource.
DIY NOS
Hysteresis and Avalanches
Bicycle Tech Site DIY CF frame building, etc.
Magnet Experiments
Speedskating Technical Articles