InterCall [IA][Direct] -
InterCall is a Mathematica package which allows easy access to external libraries such as IMSL, NAG or even your own code.
Leibniz [IA][Direct] -
Graphical front end to Mathematica which allows you to type mathematical text and carry out calculations using drag and drop manipulation.
MathSource [IA][Direct] -
An electronic resource that contains the largest collection of Mathematica packages, notebooks, examples, and programs available.