The regular expression engine in .NET is an awesome technology for text processing and string manipulation and is definitely one tool you want to have in your development arsenal.  But Regex syntax is one of the more difficult languages to get your mind around because, at first glance, it looks like a meaningless jumble of characters.  Even for someone familiar with regular expressions, it's hard to look at a Regex pattern and actually know what it does.

I'm fairly good with regular expressions, but I certainly can't remember all of the syntax and I almost never get a Regex pattern written correctly on the first try.  I got tired of trying to debug regular expressions directly in my code because it was time consuming and frustrating.  So I went out looking for a good Regex design tool, and I found an awesome one from RAD Software.  One of the best features is syntax tree that outlines all of the character escapes, substitutions, character classes, grouping constructs, quantifiers, and everything else you need when writing a Regex pattern.  It also allows you to test that pattern against a string and see a listing of all the matches it found.  It even breaks the matches out by groups. 

All in all, it's a very clean app, it's very simple to use, and it's completely free.  Check it out: http://www.radsoftware.com.au/regexdesigner/