Yours and other posts sort of hint at it, but one thing I really can't live without is Unix. The whole environment is just really conducive to me performing useful work. So much so that I have chosen a programming language (*ahem*, Perl) that tries to bottle up all the Unix goodness it can, and I usually install a Unix-like environment on any Windows machine that I have to use for any period of time (Cygwin usually does me good).