[Discussion] What crates would you like to see?

The Rust ecosystem is continually growing, but there are still a lot of areas that it has yet to cover. I find myself very impressed when I scroll through crates.io and find crates to solve problems I didn't even know existed.

In this thread, I want to ask you the simple question:

What crates do you wish existed?

Talk about them. Express what functionality you wish you could get in your code with just a simple cargo add. Maybe someone else will see your comment and know of a crate that fits that nice. Maybe someone will get a spark of motivation to build a crate that does just that.