Regular Expressions
Regular expressions are one of programmer's best friends and I believe every programmer ought to dabble with Regex in some form or the other. .NET framework provides excellent support for Regex through the System.Text.RegularExpressions namespace. Enlisted below are some great resources I recommend for creating and using Regex in .NET:
1: Jeffrey Friedl's Mastering Regular Expressions
Its one of those 'must have' books you must buy as a software professional.
2. A Regex Tutorial at CodeProject.
Great article. The best crash course on RE you'll ever find on .NET.
3. RegexLib.com : Cheat Sheet and RE Tester
A really good repository of regular expressions and a great community too.
4. RegexDesigner.NET
A simple, yet very handy tool to try out and test regular expressions.