The following applications complete very well and you should use both of them, as each one offers a different set of tweaking options.
Ubuntu Tweak is an application designed to adjust Ubuntu easier for everyone, handling package caches, tweaking nautilus, offering a bunch of application to install.
sudo add-apt-repository ppa:tualatrix/ppa
sudo apt-get update && sudo apt-get install ubuntu-tweak
Ailurus is a similar Ubuntu enhancement application. It can install/remove applications which do not provide Debian packages at all. It can change system settings. Moreover, it can detect which Ubuntu repository is the fastest one for your connection.
sudo add-apt-repository ppa:ailurus
sudo apt-get update && sudo apt-get install ailurus