Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
One of the first things I did when I moved from Vista to Ubuntu was install a nice looking theme. The default Ubuntu theme is pretty decent, but I dont like the brown and when browsing posts like this and this, I knew I wanted a sexier look for my laptop.
Where To Find Ubuntu Themes
There are a number of websites which have free Ubuntu themes for download. One of those sites that I really enjoy and get most of my themes from is Gnome-Look.org. Gnome-Look offers tons of free themes, wallpapers and icons for download. I am assuming you are using gnome as your window manager. If you are, then browse under the GTK 2 themes. Choose a theme and then we will install it below.