(disclaimer: the content of the sites linked to is the sole responsibility of their respective owners)
http://www.prosoniq.com
sonicworx...THE soundeditor, now discontinued and freeware
http://www.uisoftware.com
home of the unbelievably cool Metasynth and other apps
http://www.cycling74.com
max/msp can do just about anything
http://www.audiofile-engineering.com
samplemanager and the totally cool wave editor live here
http://www.soundhack.com
tom erbe's excellent pieces of software
http://www.nativeinstruments.de
Reaktor again...
http://www.i3net.it/Products/dspQuattro
sample-editor that can host virtual instruments
http://www.audacity.sourceforge.net
open source sample-editor
http://www.plogue.com
bidule and a couple of plugins
http://www.icedaudio.com
audiofinder is a cool tool for managing and batch-processing samples
http://www.bernsee.com
http://www.dspdimension.com
tons of DSP related stuff
http://www.osxtools.com
MacOS X related tools, including some Neuron tools
