Windows Vista only: By default, Windows Vista doesn't allow you to install custom colour and window decoration themes, unless they've been specifically reviewed by Microsoft. With a set of Uxtheme Patches, and a willingness to roll up your sleeves and patch your system, you can enable custom themes and tweak your colours, windows, and other desktop parts to your heart's content. The How-To Geek has a step-by-step guide to installing, patching, and enabling custom themes, and it's highly recommend that anyone trying it out back up their files and create a restore point before pressing on. Having said that, there are some pretty slick themes out there, at deviantART and elsewhere. Uxtheme Patches are free downloads for Windows Vista (standard and SP1) only. Image from NEXTLevel theme by patrickgs.

Windows Vista only: Windows XP users with a serious Mac crush have FlyakiteOSX to make their desktop resemble, if not work like, a Mac, but Vista users—even those upgraded to Service Pack 1—now have Vista OS X for a similar OS paint job. The installer (unfortunately) requires that you disable User Account Control to move forward, but you'll get screensavers, wallpapers, a "Dock," and more goodies, all accessible from the basic "Themes" menu. The gHacks blog states that the theme completely uninstalls itself when you're ready to move back to Windows; I haven't tried it myself, so let us know your experiences in the comments. Vista OS X is a free download for Windows Vista systems only.

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Reader David says that staring at the glowing box all day long is hard on his peepers, so he takes matters into his own hands. He writes in:

Most "Make X Look Like a Mac" hacks for any system end up merely putting a new skin on the same basic window and menu layouts. Not so with Global Menu, an Ubuntu project that's seen a lot of community input and has matured along with it. After installing more than a dozen packages and crossing your fingers, your top menu bar should emulate the OS X style, moving your program dialogs up there and keeping the application window relatively clean. At this point, the trick doesn't work with Firefox and some Java-based apps (hello OpenOffice), but add it to one previously-mentioned modification tutorial and a new Thunderbird theme, and you're getting much closer to the look and feel of a Mac, if that's your preference. Hit the link for the detailed installation instructions.

Google's personalized start page iGoogle has opened up and offered a directory of user-created themes, but not everybody can dig through XML and JavaScript APIs and come up with a working, themed page. The igThemer web application makes it easy to change every detail of your iGoogle page and save the theme for your own use or listing on the directory. The big drawback is that you can't create a page that changes with the time of day, but it leaves you with a lot fewer nits to pick if you wanted to explore the API to find out how to do that, and it's an overall handy way to customize your start page experience.

All platforms running Thunderbird: Dress Thunderbird up to look just like Mac OS X's Mail application with the Leopard Mail T-bird theme. Download the theme (right-click and choose "Save Link As") and install it using Thunderbird's Add-ons dialog box. Unlike the previously posted Apple Mail Theme, this theme works in Thunderbird 2.0, but I did have trouble getting it to look just right on my Mac, so proceed with caution. The Leopard Mail theme is a free download for Thunderbird 2.0+ only.

As seen in our iGoogle show and tell in June, personalised Google start pages can be both useful and highly customised. The folks at Google have opened the door to far more customisation and choice with the launch of an iGoogle Themes directory, as well as a guide for designing your own theme. A few new themes are already present in the directory, but many more are likely just around the corner. Those unsatisfied with the picks so far can always check out the iGoogle Skins gadget for a little DIY theming.

Linux only: Want the look and feel of a Mac without paying the high-end design premium? Tired of hearing from all your Mac-happy friends how awesome Leopard looks? Got time to run through six pages of instructions? Then HowTo Forge has got you covered. Their guide to making a nearly total theme conversion requires Compiz Fusion (installed by default on Ubuntu Gutsy Gibbon), Awn and the patience to download and place a hefty handful of files, but everything seems to be covered, right down to the system sounds. The guide is written for GNOME-based Linux systems and requires a number of downloads, some of which might not pass the most stringent legal (or open source) tests but are otherwise free. I haven't tried it myself, but this weekend's another story.