A very much partial list of organizations that produce software of use to the mathematical community.

Academic Systems (Interactive mathematics software for college and high-school)

Apple Computer

AT&T Labs - Research

Civilized Software Inc. (MLAB)

Design Science Inc. (MathType, WebEQ)

Maplesoft: Math Software (Maple) [formerly Waterloo Maple Inc.]

MathSoft Home Page (Mathcad)

MathTools Home Page (Technical computing portal)

MathTensor (MathTensor package for Mathematica)

MathWorks Home Page (Matlab, Simulink)

Mathware Home Page (Derive, Mathpert)

Microsoft

Montessori Resources (Shockwave Montessori Games)

Modelica Association (Modeling if complex physical systems)

Netlib Home Page (Repository at UTK and ORNL)

Pedagoguery Software Inc. (GrafEq, Poly)

Poliplus Software. (Smart Reader eBook)

Red Hat Linux

SAS Institute Inc. (SAS statistical products)

Scicos (Graphical dynamical system modeler and simulator toolbox included in the Scilab software)

Scientific Letter (Mathematics in e-mail)

Scilab Consortium (Open source platform for numerical computation)

Steven M. Christensen and Associates, Inc. (Consulting)

Theorist Interactive, LLC (LIVEMATH, MathEq)

Wolfram Research Inc. (Mathematica, Mathematical Explorer)