As I develop, it seems I am using regular expressions more and more. So, I am always on the lookout for a good tool to help me edit and verify my expressions.
I used to use an online tool called the Regex .NET Tester, but recently I have found a rich thick client called The Regulator. This is a free advanced regular expression editor written by Roy Osherove.
Roy has also written a regular expression creation tool for beginners called Regulazy. I haven't tried this one, but it looks like it might be a good learning tool.
In addition to the tools above I find that a good regular expression cheet sheet comes in useful every once in a while. There are two good cheet sheets located at the RegExLib and Visibone websites.