Caffeine has been a very useful app to instantly disable or enable the screensaver on mac. At least for me, I am using multiple computers in office and all of them except the one I am using will usually get idle. Getting into screensaver, however, is bad sometimes especially I was reading from that screen and type things on another screen, and not to mention the password input after the screen lock. Well anyway, I highly recommend it.
On mac, we can install it easily with the app store.
But in ubuntu, it is not available on the software center. But you can still get it through command line. Turn on terminal and input the following one by one.
sudo add-apt-repository ppa:caffeine-developers/ppa
sudo apt-get update
sudo apt-get install caffeine