[krishanath]

Here's an expression parser, written in C#, that also has the ability to execute the parse tree:

https://github.com/Rajeev-K/formula-parser

[dukoid]

Here is one written in Java: https://github.com/stefanhaustein/expressionparser

In addition to a simple math expression evaluator, examples include a BASIC interpreter and a simple computer algebra system O:)