There are many ways to customize Ubuntu to look cooler.
If you've got any cool additions, add any fairly short items to this page, and add longer items to their own pages with links from here.
To change wallpaper, right-click on your desktop and choose Change Desktop Background. Pick a wallpaper from the list or choose the Add Wallpaper button to add an image file to the list.
Use the Desktop Colors selector to change the background behind translucent or semi-transparent images - this works very well with SVG graphics.
Ubuntu has many themes, and many more can be downloaded from websites. To change themes:
Ubuntu 7.04 Development version: click System → Control Center. Then click Theme.
Ubuntu 6.10 and earlier: click System → Preferences → Theme.
Drag downloaded theme files onto the Theme Manager to make them appear in the list.
Themes can have:
Controls - sometimes called GTK Themes
Window Borders - sometimes called Metacity Themes
Most themes include all of these items, some have only one - you can find those by clicking the Customize or Theme Details button. Here you can also mix and match elements from different themes.
Changing Login Window (GDM) Themes
The login window is also sometimes called 'Gnome Desktop Manager (GDM)'.
In Ubuntu 7.04 Development Version:
Click System → Control Center → Login Window. Click the Local tab to change themes.
In Ubuntu 6.06:
Click System → Administration → Login Window.
To install new login screen themes, save the .tar.gz with your theme on it to your Desktop. In the Login Screen program, press the Install New Theme button, find your new theme's file, and press the Install button.
Then simply select the new theme from the list of available themes. You can also set it up to pick a random theme on every boot, rather than picking just one theme.
Changing the Gnome Splash Screen
Install gnome-splashscreen-manager. Start it by selecting System → Preferences → Splash Screen.
Install new splash screens with the 'Install' button by navigating to the file you want to use for your splash.
Changing Mouse Cursor Themes
Download the theme to an easily accessible location; ~/Desktop is good.
Install the gcursor package via Synaptic Package Manager or apt-get.
Start gcursor with System → Preferences → Cursor Selection.
Select a theme, or click 'Install theme' and select the compressed file to add it.
Once you're finished, select the theme you want to use and click "Close"
Getting Regular Art Updates
The Ubuntu Calendar is a regularly updated package containing this month's Ubuntu-themed desktop wallpaper. Just install ubuntu-calendar - see Installing Software, then click System → Preferences → Desktop Background. You'll find 'Ubuntu Monthly Calendar' and 'Ubuntu Monthly Calendar Widescreen' added to the list.
Gnome Art Manager
Gnome Art provides an easy way to fetch backgrounds, themes, login screen art, and more from art.gnome.org. Just install gnome-art. See Installing Software, and click System → Preferences → Art Manager.
Use the Art menu to select the category you wish to view.
Ubuntu Eyecandy Pages
Qt-Gnome: Make KDE applications look the same as Gnome apps.
Compiz - Provides smooth, 3D animated window movement, desktop switching, 'expose' like application switching, transparent terminals, and more, including simple graphical configuration tools.
GnomeLaunchBox: Mac OSX-style application launcher.
WallpaperTray: Picks a particular wallpaper each time you log in, or regularly.
Other Theme & Eyecandy Resources
BreezyCust Eyecandy Page: How-to's and tweaks