Ubuntu 12.04 has been released today and you may want to customize it. Some of the options like changing icon themes, fonts etc. are not available anymore by default. However, they can be changed using some handy configuration apps.
Gnome Tweak Tool (Available in Repositories)
Gnome Tweak Tool will allow you to change lots of different options. However, it lacks Unity specific tweaks and is more suitable for Gnome Shell.
Install Gnome Tweak Tool by running command below:
sudo apt-get install gnome-tweak-tool
The above command will pull lots of packages including Gnome Shell (if not installed), so keep that in mind.
MyUnity (Available in Repositories)
MyUnity, as the name suggests, is a config utility specially designed for Unity desktop. It has many settings of Gnome Tweak Tool like changing themes, fonts etc plus extra options for tweaking Unity desktop. If you are using Unity only, go with it.
Install MyUnity by running command:
sudo apt-get install myunity
Ubuntu Tweak is easily the one of the best applications for customizing Ubuntu. It has many more options then MyUnity, including recent support for App Center, Source Center, PPA Purge, Quicklist Editor, Login Settings etc.
Install Ubuntu Tweak by running commands below:
sudo add-apt-repository ppa:tualatrix/ppa
sudo apt-get update
sudo apt-get install ubuntu-tweak
Unsettings is another tool similar to MyUnity for Unity desktop. It has less features than Ubuntu Tweak but some of the newest unity features like disabling HUD, Overlay Scrollbars, Global Menu etc. are available in Unsettings.
Install Unsettings by running commands:
sudo add-apt-repository ppa:diesch/testing
sudo apt-get update
sudo apt-get install unsettings
MyUnity, Ubuntu Tweak and Unsettings, each of these tools can restore default settings in case you mess up. With these apps, you can comfortably avoid using Compiz Config Settings Manager (CCSM) which is more dangerous and advanced and can severely break your system if you apply wrong settings by accident.