A long-time satisfied user of LINQPad. It is one of my must haves. I usually upgrade to the latest one when I hit some framework feature or a piece of snippet which requires it. It started as a very reasonably priced for the paid edition and there was only one paid edition for a long time. Nowadays, it is a bit pricier and has several editions (a suggestion would be to merge Pro and Developer editions).

I use LINQPad to manage my collection of snippets to do some ad hoc tasks on my machine. I also use it for testing out new code and learning about new features. It would be great when it becomes cross-platform, but last I heard Jo Albahari mentioned that the investment required was quite big and not worth it (at that time).

On the side note, I had a chance to see Jo Albahari's presentation few years ago in Sydney and I think he is unsung hero of .NET world (along with Ayende)!