Audio Links

Software Tools:

Holm Impulse - Free swept sine acoustic measurement program
Hornresp THE horn simulator. Here's a massive thread on details about Hornresp.
Unibox Excel sheet for doing T/S box design.
FEMM Magnetic finite element simulation program.
LTspice Helpful Spice program for designing active filters or any electronic circuits.
Fusion 360 3D modeling, FEA, and CAM program that is free for hobbyists or non-professionals.

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.
Madisound A great source for speakers. Very good service, in my experience.
Parts Express Another good source for speakers and other parts.
DIY Audio An audio discussion forum.
MiniDSP Makers of DSP gear for hobbyists.

Other fun stuff:

The Scientist and Engineer's Guide to Digital Signal Processing An easy to read introduction to DSP.
CNC Zone A CNC discussion forum.
Mr. Money Mustache A blog about early retirement / financial independance.
Tastytrade A 'financial network' where they talk about how to trade options (for free).
John Broskie's Tubecad page Tons of ideas and information on tube projects
Eric's World of Physics Tons of information. Comes in handy sometimes. Other times it's a bit cryptic.
DJ Aerotec
Vacuum Bagging Wings
Vacuum System Project
A Vacuum Gauge
Foam Cutter Links
Backyard Metal Casting
Material Properties - this is an awesome resource.
Hysteresis and Avalanches
Bicycle Tech Site DIY CF frame building, etc.