Looks like Ubuntu Tweak project is no longer maintained and the application is no longer available through PPA. I particularly loved the Janitor feature in Ubuntu Tweak. Are there any alternatives one can use?
Source - Ubuntu Tweak in Launchpad
if you are familiar with the command line you can do exact the same thing yourself with a look at this answer: https://askubuntu.com/a/234792/354350
BUT be careful and you should know what you are doing, because some commands use superuser-rights!
...maybe write a little script and everything is done with just one click! ;-)
anyway i would recommend to use BleachBit: it's a good file-cleaner with a GUI and (also in superuser-mode) you can hardly mess up your system.
open your terminal and just type
sudo apt-get update && sudo apt-get install -y bleachbit, type your password (no characters will be displayed) and wait till executed.
You can use
unity-tweak-tool to install it:
sudo apt-get install unity-tweak-tool
If cleaning is your objective, you have many options. Bleachbit is great. Nautilus has addons [use Synaptic to install these]. In terminal, you have autoremove and autoclean.