Analytica: Analytica provides consulting and training to the Mathematica community.
HostSRV: A web hosting company dedicated to providing MathScript capabilities to websites.
MathC: MathC provides statistical consulting using Mathematica, visualization of mathematical concepts in the area of teaching Mathematics, and Mathematica-applied college-level mathematics.
MathTensor, Inc.: Developers and distributors of MathTensor, a Mathematica-based package for performing tensor analysis by computer. Formerly MathSolutions, Inc.
Scientific Arts: Consulting with Mathematica: Scientific Arts is a consulting and creative services company which provides targeted solutions for a broad range of scientific enterprises.
Wolfram.com - Mathematica: Symbolic, graphics, language and numerical software. Page includes explanation and visual snapshots of program.