Well, I'm too busy nowadays with school to write an exhaustive prettyness thread, but I can get things rolling with a few basic instructions.
I only have experience prettifying Gnome, so that's what I'm going to write for.
Ok so you've installed the latest Gnome distro...this means you've either installed Fedora or Ubuntu. Both of these distros are embarrassingly ugly out of the box. To make my desktop look good, here's the process I usually follow:
Step 1) Change the background.
Everyone has their own background site. http://www.deviantart.com/ is a very popular site where you can find any number of backgrounds. Find a picture you really like, and then check out more by the same artist....usually you find all kinds of stuff this way. Additionally, there's http://gnome-look.org and http://art.gnome.org/ . Both of these site specialize in Gnome artwork. You're bound to find a background you like.
Once you find a background you like, download it to your computer. Then right-click on the desktop (anywhere that's not a window, menu, or icon will do), and select "Change Desktop Background." Add your new file by selecting "Add New Wallpaper" and navigating to the location of your new picture. There you go. Clicking on the new wallpaper will enable it.
Step 2) Change the GTK and Metacity theme.
Also called "Application" and "Window Manager" themes. GTK is the widget set used to draw windows in Gnome. It's responsible for things like buttons, menus, scroll bars, etc. Bluecurve (Fedora default theme) is alright. Human (Ubuntu default theme) is a visual abortion. I'm not sure how something so ugly came to be the default face to present to the user, but one way or another it has to go. Metacity is the window manager for Gnome. It's the bar you see at the top of every window that has "close", "minimize", and "Maximize" buttons in it.
The best sources of GTK/Metacity themes I know are http://gnome-look.org and http://art.gnome.org/ . Browse around and find one that looks nice and goes with your background (I can't stress this enough...make sure the colours in your background and gtk themes complement each other. I see too many instances of people who pay no regard to how things work together, and the end result is always a disaster). Once you have a theme picked out, download it to your computer. Now navigate to System->Preferences->Theme (must have at least Gnome 2.10...if you don't have 2.10, find the Themes application). Click on "Theme Details" and a window will pop up. Drag the file you downloaded to the new window. This will install the contents of the file you downloaded into your ~/.themes directory. You can now select the new theme from the window you just dragged it to. The window defaults to a list of GTK themes, but there's a tab at the top you can click to see your metacity themes.
This method of downloading and dragging works 95% of the time. In the event that the author doesn't know what they're doing and hasn't bundled the theme up in the way that Gnome expects, you can usually still install the theme by extracting the archive (right-click on the file you downloaded and select "Extract Here", and copying the folder that contains a file called "gtkrc" to your ~/.themes directory. For example, if I have a folder hierarchy like "Clearlooks/ClearlooksBlue/gtkrc", I'd copy the "ClearlooksBlue" directory into the ".themes" directory located under /home/myusername. Remember that this is only a backup in case things fail by the first method.
It is also worth mentioning that http://art.gnome.org has a graphical frontend that you can install on your computer. You can get it from http://www.miketech.net/gnome-art/index.php#download . I've been using it for some time, and it works very well. It's outrageously cool, and really simple to use. This is how desktop theming should be done.
Step 3) Change your icons
This step is a little more complicated if you're not comfortable with things like folders and directory structure. You see, for whatever stupid reason, Gnome stores window manager (Metacity) themes and GTK themes in the ~/.themes directory, and icons in the ~./icons directory. There's a very long-standing bug in the theme manager in which icons are installed into ~/.themes and as a result, they're not picked up by the theme changer. In order to work around this, you must manually install the icons to ~/.icons.
Get your favorite icon package from http://art.gnome.org/ or http://gnome-look.org/ . Download it to your computer, and extract it by right-clicking on it and selecting "extract here". Open up the folder that's created, and navigate to the directory containing "index.theme". Copy this directory to ~/.icons. For example, if I have the directory structure "SuedeIcons_FILES/Suede/index.theme", I'd copy the "Suede" directory to ~/.icons. This will give you access to your icons via the theme manager.
Step 4) Desklets
Ok so now we have a sexy background, a nice GTK theme, a pretty metacity theme, and some good looking icons. It's time to get some eye candy on the desktop itself. There's a program called "GDesklets" that installs transparent utilities on your desktop. You can see gdesklets in action, as well as download the program and the desklets themselves over at http://gdesklets.gnomedesktop.org/ . Install the utility (probably available by apt-get or by yum as well), and start it by typing "gdesklets". Note that if you're using X Composite Manager, you'll have to start it by typing "gdesklets --translucent" otherwise all your desklets will appear in black boxes. More on this later.
I don't run a whole lot of desklets, so the installation of desklets is kind of left as an exercise to the reader. To the best of my recollection, if you right-click the gdesklets icon in the taskbar (looks like a blue square with puzzle pieces in it), and select "Manage Desklets', a window will pop up. I believe you can drag desklet archives into this window and install them.
Step 5) X Composite Manager
This is a must for anyone who's serious about eye candy. Unfortunately, it's a relatively new technology so it's still kind of unstable, and a little difficult to get working well.
The Composite extension to Xorg allows you to do stunningly gorgeous things such as drop shadows and true transparency in windows. If you've ever seen Mac OSX with its drop shadows and translucency, it's the same kind of deal. It will make your linux desktop look easily as good as a Mac, and WAAAAAAAY better than anything Windows can offer.
The setup of the composite extension is too involved to cover in this article. It depends on your video card, and is a multi-step process. You can find my quick-n-dirty howto here http://www.linuxjunior.org/yabbse/in...d=9794;start=0 . This page also includes a link to a more thorough tutorial.
Trust me...if you can get x composite manager up and running, you do want to. It adds a whole new element of depth and beauty to your desktop, and makes the difference between something that people just dismiss as an ok-looking linux desktop, and something that makes people do double-takes (and in many cases, absolutely lose it over how good your environment looks).
And that's it. Slick background + nice metacity + nice gtk + nice icons + your choice of gdesklets + xcompmgr = drop dead gorgeous linux desktop!