This plug-in can be used to evaluate a (free-form) mathematical expression based on the input attribute values. The computed results will be returned as a float on the output attribute and can be connected to any other node in the Maya DG. Commonly used functions are shown in the list "Mathematical Functions" in the Python help. There are also some constants available, such as pi (the ratio of the circumference of a circle to its diameter), e (the base of the natural logarithms), etc...
You can also include limited python scripts and functions on the evaluated expression as long you use an expression which can be interpreted as a Maya Python script and follows the DG guidelines for writing nodes.
This plug-in can be used in place of the Maya 'Expression' system. Maya's evaluation of 'Expressions' depends on the contents of the expression – sometimes you have to force evaluation - whereas, for a node, the Maya DG will always do it for you when needed. Debugging a Maya 'Expression' is not easy either (no tools are available), and requires a knowledge of MEL. Using the MathNode plug-in, you can use a Python debugger to debug the Node evaluation (see the Blog post for instructions on how to do that. In any case, it is always preferable to use self-contained nodes and connections over expressions whenever possible.
Available on Windows, Mac OSX, and Linux