The biggest worldwide community of Scientists and Practitioners with an interest in Operations research and quantitative methods to improve business decisions. Very informative website!
If you are writing mathematical texts, you will sooner or later come into contact with TeX/LaTeX, a document typesetting system that simplifies the entry of formulas and structured documents. The TeX User Group (TUG) is the American organization of users of this system. Most of this software is available for free!
The manufacturer of MatLab (short for Matrix laboratory), an invaluable tool to test and get numerical results for new algorithms.