While working on the systems team at Automattic, I became closely acquainted with our  server deployment toolkit, an in-house yet GPL’d software named ServerMattic.

I find it to be not only elegant in its simplicity, but also brilliant when it come to learning curve and flexibility. Why? Simple: all in all, it’s just a set of clever shell scripts, backed by SVN, running more shell scripts. This means that anyone familiar with even the basics of Linux/Unix systems administration can wrap their brain around it in no time. On the other hand, it’s also powerful enough to maintain over a thousand servers with ease and sanity.

See http://code.trac.wordpress.org for more non-WordPress open source software by Automattic.