Two arbitrary picks:

https://crates.io/crates/serde

https://crates.io/crates/regex

Anything covered by Gjengset's "decrusted" series: https://youtube.com/playlist?list=PLqbS7AVVErFirH9armw8yXlE6...

Sort of on the border between toy and not-toy; Gjengset implements a concurrent hash map: https://youtube.com/playlist?list=PLqbS7AVVErFj824-6QgnK_Za1... [17hr recorded over 3 streams]