I mentioned Expresso, a regular expression tool written by Jim Hollenhorst, on my blog back in February along with several other regular expression tools and resources. Version 3.0 is now in Beta and is well worth a look.
Expresso is not only a great tool for building and testing new regular expressions, but it’s also great for understanding existing expressions, as it provides an explained English breakdown of the various parts of an expression as a tree. It’s rapidly becoming my favorite regular expression tool.