Themes
The general look & feel

http://www.gnome-look.org/
Good resource for different themes, etc.

You'll want either GTK2 themes, or Metacity themes

Metacity just deals with the window borders (inc. top title bars of windows)

GTK2 themes do the whole kaboodle
They'll theme any combination of:
- Window borders (inc. top title bars)
- Panels
- Icons
- Wallpaper
- Login screen
- Startup / shutdown splash screens

Download a theme (it'll be a tar.gz file)
open up the appearance preferences (System > Preferences > Appearance)
Drag & drop the theme file into the Theme window


Compiz Effects
The 3D desktop stuff

In addition to themes, you can change how your desktop operates
including so called 3D effects
You'll need to have a GPU accelerated graphics card for this

Enable Compiz
System > Preferences > Appearance
under the "Visual Effects" tab, set to normal or extra

Install the compizconfig-settings-manager
Open up Synaptic
System > Admin > Synaptic
Search for the compizconfig-settings-manager
Install

compizconfig-settings-manager now under System > Preferences
you can play all day with the different plugins & settings