I mean serde is – in my understanding – the most useful crate out there. It does exactly one thing and that very well.

I love chumsky and iced. They are still under heavy feature development, but their design feels great.

Here is an originally random list (using cargo tree --prefix=depth) with some very loose logical grouping. Wide-scoped and well-known crates removed (some remaining are probably still known by most).

mime data-encoding percent-encoding textwrap unescape unicode-width scraper
arrayvec bimap bstr enum-iterator os_str_bytes pretty_assertions paste
clap_complete console indicatif shlex
lz4_flex mpeg2ts roxmltree speedy
aes base64 hex cbc sha1 sha2 rsa
reverse_geocoder trust-dns-resolver
signal-hook signal-hook-tokio
blocking
fs2
semver
snmalloc-rs

If you are looking for something specific or a category of crate you may want to checkout lib.rs, a great alternative frontend for crates.

I like isolang for handling ISO-639 language codes.

(Disclaimer: I contributed to it)

In the async world, I love the smol stack where each functionality is split into an independent subcrate.