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There are several tools that can assist with creating and testing regular expressions. The Regulator and Expresso are perhaps the most widely used.

The Regulator, written by Roy Osherove, features syntax highlighting and integrates with Regexlib.com's database of online regular expressions via a web-service.

Expresso, written by Jim Hollenhorst, is another great tool for building and testing regular expressions. Expresso has some very nice features such as generating ready-to-paste C# code snippets from your regular expression.

Regex Workbench, written by Eric Gunnerson, has a nice feature that shows Tooltips that decode the meaning of subexpressions within an expression.

RegexDesigner written by Chris Sells is another useful tool. It is not quite as fully featured as some of the others.

Resources

• Regexlib: Regular Expression library: Regexlib also has a good resources section.
• Regular-Expression.info : has very clear explanations of various regular expression constructs, and some good examples.
• Learning .NET Regular Expressions with Expresso
• Introduction to Regular Expressions in .NET
• Regular Expressions in .NET by Darren Neimke.
• 4GuysFromRolla Regular Expression Information Roundup: includes links to introductory articles.
• Common Applications of Regular Expressions by Richard Lowe

Regular Expression Cheat Sheets

It might be a strange name for a technical site but www.ilovejackdaniels.com has a few gems of ‘cheat’ sheets (memory aid would be more accurate). The excellent regular expression sheet is no exception. In addition to the downloadable and printable cheat sheet this page gives one of the most understandable descriptions of regular expression functionality you will find anywhere.
There is a slightly less comprehensive but useful crib sheet here.

UPDATED: Add Regular-Expression.info to the resources list.