There is a very handy buildbot service at http://buildbot.rust-lang.org/. It, guided by bors, builds all reviewed pull requests, and merges them if tests pass.
The current platforms are:
Linux (Ubuntu 12.04 LTS and CentOS 6, x86 and x64)
OS X Lion (10.7.4, x86 and x64)
FreeBSD 9 (x64)
Windows Server 2008R2 (x86)