
Linqpad allows you to write little snippets of code and immediately see the results. I use it often to test out regular expression patterns and to test out features of C#/.Net that I haven't used previously to see how they work. Intellisense (paid version) is a life saver too. Review collected by and hosted on G2.com.
The only thing that I dislike is not having the ability to add nuget packages for the free version. I don't think that should be only offered to paying users. Review collected by and hosted on G2.com.