Over the course of the past year I've become a huge fan of Apple's Mac OS X on the server-side. Learning about and taking advantage of some cocoa programming fu, like CoreImage.
Of course, I'm primarily a PHP programmer, not that I couldn’t hack around in cocoa, I just haven't had much time to learn more about it.
mod_rewrite is a very useful, and very complicated Apache module that lets you manipulate incoming requests. Uses for this module vary from redirecting content from one URL to another for the purpose of maintaing SEO and legacy URLs to simply making your URLs more readable and understandable, and not so weighed down with technical luggage, as it were.
On Mac servers I love the advent of a neat feature called Time Machine. Time Machine does incremental backups without having to deal with 3rd-party software, or custom shell scripts like the one I'm about to show. Unfortunately, on the Linux side there's always some degree of elbow grease that's required. Which isn't to say you couldn't have Time Machine-like backups on Linux, you certainly can, it just takes a more concentrated technical effort, and you don't get the nifty GUI.