LINQPad by Joseph Albahari is one of the most useful tools I have ever had the pleasure of using. If you are using or learning C# 3.0 and/or the new LINQ syntax it is invaluable.
LINQPad was created to support C# 3.0 in a Nutshell by Joseph Albahari. It provides an interactive means to test snippets of C# 3.0 code and LINQ against arrays, XML and SQL Server databases. Download it now and join the challenge to use it for a week instead of SQL Server Management Studio and you will find that you will be able to replace 80% of what you do in SSMS with LINQPad and while doing that you'll be using LINQ and C# to do your queries in a more interactive and intuitive style.