LinqPad

LinqPad is a subsystem interpreter for LINQ in the same vein as:

• regular expression tools
  like RegexDesigner.NET or Expresso or RegexDesigner
• C# snippet compiler
• ADO.NET CSV file tester
• .NET Format Designer

It allows you to test out all the LINQ features in .NET 3.5 against any data source of your choosing, plus it provides lots of test cases and sample data sources. I recommend LINQ Pocket Reference to go with this.