User Contributed Free Modules for Matlab/Mathematica/Maple

Matlab, Mathematica and Maple are not free softwares. But a great
advantage of using these commercial packages comes in the form of
user-contributed modules and other useful resources.

Wolfram's MathSource is a part of the Mathematica Information Centre.
It has over 750 Mathematica packages and programs categorised over
various sciences and sub-topics, like Applied Mathematics, Information
Science and Technology, Arts and Humanities, Mathematica Technology,
Business and Economics, Mathematics, Education, Engineering and even
Social Science.

Mathworks similarly have thousands of M-files in 'Matlab Central' for
various applications. If you are trying to write some complicated Matlab
script on your own, it is always a good idea to check Matlab Central.

Similarly, Maplesoft provides the Maple Application Centre likewise for
Maple applications.