[upthedale]

Try Linqpad for a scratchpad too. Don't let the linq name fool you - its great for general purpose fiddling about. It has been truly indispensable when developing on .Net.

http://www.linqpad.net

[ygra]

I usually use PowerShell for fiddling which is often nice enough for trying out things with the BCL.

[taude]

Back when I worked with .NET and Linq, I was a fan of this tool