See also this list of publishers.

Academic Press

Aces Research, Inc. (Developers of Mathematics Library Plus)

Addison-Wesley Interactive

Adept Scientific

Advanced Scientific Applications Inc. (SciMath)

Anton Textbooks, Inc.

Artlandia, Mathematica-based software for creating mathematical art

Basic Research Institute in the Mathematical Sciences

Birkhauser AG

Brooks/Cole Publishing Mathematics Page

Chicago Lectures in Mathematics

Compass Modeling Solutions, Inc.

The Computational Mechanics Company, Inc (COMCO)

CPLEX Optimization, Inc.

Cross Educational Software

Elegant Mathematics, Inc.

Elsevier Publishing

ExcelMath (AnsMar Publishers, Inc.)

Floating Point Software (ExacMath)

GAMS Development Corp. (modeling)

International Press

Harmonic Software Inc. (Makers of O-Matrix)

Key Curriculum Press

Khayyam Publishing Company, Inc.

Learning in Motion

Lemma Inc.

Logix Consulting

Mandalay Scientific Inc. (Data Modeling)

Mathematica (wri.com)

MatheMatrix, Inc.

MathMedia Educational Software, Inc.

MathPlus

MathPro Press

MathSoft Home Page (Developers of Mathcad)

MathSolutions, Inc.

MathType

MathWare (Derive)

Mathtools.net

MathWorks Home Page (Producers of MATLAB)

Minitab Inc

Neufeld & Associates

Numerical Algorithms Group Ltd

OPCOM Pty Limited

Pedagoguery Software

Polymorf

Principia Consulting

PWS Mathematics

SGI Operations Research Initiative

Soft Integration, Inc.

South-Western Educational Publishing Mathematics

Springer

Springer New York

Steven M. Christensen and Associates, Inc.

TCI Software Research

Thompson Publishing

Thompson Publishing Math Catalog Search

TSP International

Visual Numerics

Waterloo Maple Inc.

West Educational Publishing Mathematics and Statistics

XOX Corporation