Hopefully you all already know about the most excellent LinqPad app. I've used it for the longest time, mainly to easily open up SQLCE database. Yes, I know, SQLCE is very much end of life, but I've done a LOT of testing with it over the years since it's been available in Umbraco, as an easy way to do quick prototyping while testing reports on the issue tracker.
LinqPad
But there's a lot more that LinqPad can do! It's a mini IDE as well, super interesting for rapid prototyping. I know we can do a some of this stuff online at Try Dot Net as well but I often want to try things that need a NuGet package to be installed and then there's the magic Dump extension method that is advertized as: "LINQPad’s Dump method is famous for its capacity to eat almost anything!"
LinqPad is free, but personally I've found it worth paying $59 for the Pro edition, mostly to get autocomplete and tooltips in my code, making my rapid prototyping even more rapid!