note2.com

Dspguru.com
Site of the comb.dsp newsgroup with lots of info and technics.

Musicdsp.org
Archive of code and techniques from the music dsp mailing list.

http://shoko.calarts.edu/~glmrboy/musicdsp/music-dsp.html
Site of the music dsp mailing list. Includes reviews on a collection of books for audio and general signal processing. Many links too (not all working though).

Dspdimension.com
Site of Stephan Sprenger with tutorials (and code) on pitch shifting and time stretching.

http://www.cs.princeton.edu/~prc/
Perry Cook's site and home of the STK (Synthesis ToolKit) library.

Dspguide.com
Home of Steven W. Smith's book The Scientist and Engineer's
Guide to Digital Signal Processing, an introductory DSP book that is also freely
available!

Vst code archive
Information and code on how to develop vst plugins.

Tobybear
Information about how to develop vst plugins using Delphi +source code and utilities.

Tutorial DIY@Computer music mag
Step by step and from the ground up, program your own Plug-In...

Introduction to Signal Processing by Sophocles J. Orfanidis
C and MATLAB Functions

DAFx'05 Conference Proceedings
Papers of the DAFx'05 conference proceedings in pdf format (original published in a 323 pages volume and in a CD-ROM).

Evaluation of Modern Sound Synthesis Methods by Tero Tolonen, Vesa Valimaki and Matii Karjalainen
University of Helsinki report (pdf)