Software
Basta! Loudspeaker Simulation Software 
Basta! simulates open baffles, closed boxes, vented boxes and ported bandpass systems. It also includes simulation of baffle step, active and passive crossover filters and room gain approximation.
AJHorn TML and Horn Simulation Software
AJHorn is a calculation program for any loudspeakers, especially Horns and Transmissionlines.
LTspice IV is a high performance SPICE simulator and schematic capture including models for easing the simulation.
FreePCB Printed Circuit Board Editor
FreePCB is a free, open-source PCB editor for Windows. It was designed to be easy to learn and easy to use, yet capable of professional-quality work.
LibreCAD is a free, open-source 2D CAD application based on the community edition of QCad. LibreCAD has been re-structured and is available in more than 20 languages and for all major operating systems.
Exact Audio Copy is a free audio grabber for audio CDs. It works with a advanced technology, which reads audio CDs almost perfectly.
SoX is the Swiss Army knife of sound processing programs. It is a command line utility that can convert numerous formats of computer audio files in to other formats and also apply various effects to these audio files.
Foobar is an freeware audio player for the Windows platform. It is undoubtedly one of the lightest, fastest and most functional audio players.
Audacious is an open source advanced music player that is lightweight and focused on delivering excellent audio quality while supporting a wide variety of codecs. It runs on Linux, on BSD derivatives and on Windows.
Mp3tag is a powerful and easy-to-use tool to edit metadata of common audio formats. It can rename files based on the tag information, replace characters or words in tags etc. and also supports online database lookups among many other things. In a word, it is the ultimate audio tag editor.
Audacity is a free, open-source, cross-platform software for recording and editing sounds.
Forum
diyAudio is a DIY audio community to learn, share knowledge, and enjoy interacting with others interested in the design and construction of audio components.
Audio Asylum is a community for audiophiles. There you can ask questions and get advice on audio equipment. It is also a special place for music lovers.
DIY
Art Ludwig - Art Ludwig's Sound Page
Shinichi-Kamijo - Evolve Power Amplifiers
Claudio Bonavolta - Give life to music, use tubes...
Nelson Pass - DIY Power Amplifiers and Preamplifiers
Jean-Marc Plantefeve - Amplification audiophile et DIY
Siegfried Linkwitz - Loudspeakers and electro-acoustics
Components
Michael Percy Audio - Parts for audio hobbyists
Diy HiFi Supply - Components for your high-end audio
Audiophonics - Parts and Products for audio hobbyists
Hifi Collective Ltd - Ultimate components for hi-fi diyers