Um, Most of the links in this sticky are extremely old and outdated. So I'm going to start posting current links. Restoration time. Beryl Theming site
As the link says, has info and themes for the Compiz and Beryl as well as Fusion. -Compiz Fusion Forum
Which is always useful for tips and tricks with dealing with this new customization software. Also a great place to get apps and scripts that you'd otherwise never hear about. Gnome-Look.org
-Primarily a Gnome site, but worth a look if your into Linux customization.Gnome-look.org
-Primarily a KDE site, sister site of Gnome-look. Xfce-look.org
-For those that prefer Xfce, this is a great customization and theming site for you. KDE-apps.org
Great KDE customization site.OpenDesktop.org
Newly released site for general linux customization DeviantArt's Linux section
Various themes and icons. just click on the categories menu for the list of stuff.
Basic list, if you have anything else, post it!