I use LinqPad[1] as my C#/F# scratchpad. Works wonderfully well and I can't imagine living without the magic ".Dump()" feature.
[1] - http://www.linqpad.net
[1] - http://www.linqpad.net
Note: I've tried to use scriptcs, but the extent of my C# knowledge consists of pretending it's Java. But people do seem to compare it to LINQPad, so maybe it fits.