The current dev branch. This is the most up to date version with the latest bug fixes and features.
Presently, the easiest way to install the nightly binaries for Linux and Mac is to run this in your shell:
$ curl -s https://static.rust-lang.org/rustup.sh | sudo sh
This will also install Cargo, Rust's package manager, which the other installers do not.
The latest release. These are made roughly every three months and become out of date quickly.
Additional resources – including release notes, signatures, previous releases.