To add to the list, my favorites are:

* cargo-expand [0]: dumps the expansion of troublesome macros

* cargo-bloat [1]: explains why a package is so huge

* ripgrep [2]: RIP grep

* llvm-tools-preview (via [3]): the LLVM tools with which Rust was compiled (because LLVM versions sometimes don't interoperate)

[0] https://crates.io/crates/cargo-expand [1] https://crates.io/crates/cargo-bloat [2] https://crates.io/crates/ripgrep [3] https://crates.io/crates/cargo-binutils