QuickREx is a regular-expression plugin for Eclipse. It features on-the-fly evalution, code-completion, group-navigation, different engines and a library for regular expressions. QuickREx also runs as a standalone application - check the downloads.
simple and straight forward to use
This tool really makes the life of a regular expression developer much easier. Enter various input in the text box and get quick turn-around on what your expression matches and doesn't match. Superb!