Dot Emacs
Emacs is almost infinately customisable; a fact I didn't really get to grips with this fact untill I found myself on a Fedora Core box a while back with a net connection but without my favourite set of keyboard short cuts. This really brought home how just a few simple customisations can come to be relied upon.
Determined never to be caught short in such a manner again I decided to make my .emacs permanently available, providing I have a net connection that is.