LINQPad

LINQPad is a free lightweight tool that allows you to test code snippets for C#/F#/VB quickly! This saves you the trouble from opening another instance of Visual Studio and create a new solution to test your code or maybe quickly test it in a block of code in your existing application. In this last case it could take an insane amount of time to navigate to that code and/or to compile the large application.

Some other useful features are converting LINQ to SQL and analyze the performance of your code. If you write a lot of LINQ queries, then this tool is a must have as it allows you to test your LINQ or SQL queries easily!

LINQPad is available for free and has additional features for those that need more, although the free version should be enough for most of us!