Functal is a .Net library for writing computational expressions, and fills the gap between a scientific calculator and a full on scripting language. You can use Functal to do things like:

Writing a graphing tool for user-defined formulae.

Create apps with customisable relations between entities (like spreadsheet formulae).

Smart input fields, where someone can type things like Pi*3/2 instead of breaking out the calculator.

Functal goes beyond just mathematical expressions, with lots of great language features like conditionals, types and more. It's designed for efficient execution, and it's easy to use.