AFAIK, all official Rust project repos can be found in the rust-lang organisation on GitHub. Other official crates would be futures and libc, for example. Or even better, you can look at the crates on crates.io that are owned by the rust-lang-owner account.
Note that being maintained by rust-lang does not necessarily mean they're the best crates or even the best maintained crates. Sometimes they are there simply for historical reasons (e.g. rustc used them before there was an alternative).