Caffeine is an app indicator that allows Ubuntu users to disable temporarily their screensaver and the "Sleep" mode. You can even enable Caffeine for a specific running program.
To install Caffeine on Ubuntu 11.04/10.10/10.04, run these commands using the Terminal:
sudo add-apt-repository ppa:caffeine-developers/ppa
sudo apt-get update
sudo apt-get install caffeine
Use then the dash to start Caffeine:
Once launched, click its icon and select Disable Screensaver to prevent your computer from running the screensaver:
If you want to make Caffeine start automatically after login or you want to activate Caffeine for a specific running program, then open the Preferences window: