I’m going to say kbknapp’s Clap crate - I have never, in any language I have ever worked in, had an argument parsing library that was so completely painless.

I’ve found it especially nice for mocking up the skeleton of a tool where all roads lead to panic!(), then splitting it up further and further, pushing the panic!()s down the branching logic of what to actually do, until a whole utility has appeared from nowhere.