The current dev branch. This is the most up to date version with the latest bug fixes and features.
An easy 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
An alpha build of the upcoming 1.0. This is a feature-complete preview of the upcoming 1.0.0 release. Until the first beta release it is still recommended to use the nightlies.