Sjöström Audio

http://www.hificollective.co.uk
DIY stuff such as Tent shunt regulators, loudspeaker elements, tube amps. products from NewClassD such as regulators, discrete opamps, soft start.
http://www.fguenther.de/kakteen/links.htm
Link collection cacti web sites. Most of the links are to german web sites.
http://www.kukaudio.com/au/index.asp
Kuk Audio Co., Ltd is highly specialized in supplying a variety of high-quality components for audio amplifiers. They sell many kinds of amp-,audio-,guitar parts and others parts like this, for instance: Vacuum tubes,Tube sockets, Capacitors, Resistors...
http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf
A very well written book about opamps operational amplifiers and their applications. 464 pages loaded with good information, all for free!
http://www.phi-audio.com
High-End hi-fi company based in South Africa with a focus on fine workmanship and a bent for the minimalist.
http://marcusgun.com/elektronik.html
This swedish gentleman has done a nice power amp with simple means, no CAD for instance. Hand drawn schematics and hand taped pcb layout.
http://www.hpmuseum.org/
This website has everything about Hewlett-Packard's calculators. Check out the Java based simulation of a HP calculator. Cool!
http://www.quadesl.org/
This web site is dedicated for the brittish and classical electrostatic loudspeaker from Quad.
http://www.transmissionaudio.com
Bo Bengtsson is my hero, from Sweden and my hometown Eskilstuna. His company makes very nice speakers.
http://www.weiss.ch
Ultra High Performance Audio and the famous Samuel Groner works there. The products look really nice too.