Years ago, when I made the permanent switch from Apple OS X to Ubuntu Linux, the most difficult part was living without two of the tools (non-Apple ones) that made using a Mac so much more tolerable: Quicksilver and Growl. Now that their equivalents on Linux have reached a point of relative maturity that makes that easier to bear, I realize that there are many, many, many more tools on Linux that I just couldn't live without, if I ever had to switch back (at gunpoint).
The most pervasive of these in my everyday workflow is Compiz-Fusion, which is not only the sexiest eye-candy available on any OS, but has single-handedly changed the way I manage the workflow on my computer. The new cylinder-desktop plugin only pushes the boundaries further.
I am very, very picky when it comes to interface - having been weaned on sci-fi, video games and virtual reality, the mouse and keyboard desktop just doesn't cut it when compared to Iron Man's Jarvis, or Tom Cruise's Minority Report crime database. After booting Windows and playing Mass Effect for a while, or navigating the jaw-droppingly beautiful menu system of Grid (or any Codemasters game, really), it's a very rough awakening to quit the game and land back in ugly Windows world.
So far, the closest thing I've experienced to all this dreamy, futuristic, transcendent goodness on a real computer has to be my Linux laptop with Compiz-Fusion (and a whack of other tools) enabled. I've grown increasingly dependent on not only the visual representation I'm able to assign to my ideas and information, but on the ease with which I can switch midstream between them. The fact that, esthetically, it's the hottest thing ever to grace a desktop screen, is only a bonus. In fact, of the couple of dozen or so people I've convinced to switch to Linux, almost all of them did so after I showed them how I use my desktop.
Back when I used a Mac, trying to use someone else's Mac that didn't have Quicksilver felt very tedious. It felt like the computer was broken. Now, trying to use a computer without Compiz feels the same. I brought an old laptop with me when to Colombia last year that didn't support Compiz, and I paid the price in productivity.
This isn't an OS rant, either - I actually have all three major operating systems (Linux, Vista, and Kalyway's OS X) running on my laptop workstation. I'm in Linux 90% of the time for work. Once in a while I'll boot Vista to play a game (although this is less and less necessary since Linux runs most of my favorite games decently through Wine), and OS X a little for testing, and to mess around a bit with Garageband and iWork Pages. For the actual operating systems themselves, pragmatically, I'm pretty OS independent. Ethically, it's a different story... but if you look at my desktop, you'll see influences from my time with OS X such as my dock, and inspirations from Windows such as my window decorations. The difference lies in the freedom to make things yours, though - unlike OS X, I can customize my dock as I see fit, and I have my window decorations just so. For someone like me who's especially schizophrenic about the way his desktop looks and is always changing this and that, my current desktop setup hasn't changed for the simple reason that everything works together so effin' well. The only change I've made in a while is the aforementioned switch from the Cube multiple desktop plugin to the cylinder (essentially the same, but round :p ).
Freedom really can be beautiful... :D