Caffeine is a tool used to temporarily prevent the activation of the screensaver / lock screen / sleep mode when using full-screen windows. The application is useful when using video players that don't do this automatically, when listening to music while not using the computer, etc.
The latest Caffeine version (2.7.x), released back in May, no longer comes with an Ubuntu AppIndicator so you can't manually toggle it on/off any more, feature which was considered pretty important by many of its users. If you're one of them, you can now use Caffeine Plus, a Caffeine 2.7 fork which restores the Ubuntu AppIndicator icon, allowing you to manually toggle it on/off.
Like the original Caffeine, the fork also inhibits the screensaver / lock screen automatically if it detects a full-screen window.
Update: with the 2.8 release, the original Caffeine got its indicator back!
Install Caffeine Plus in Ubuntu 14.04 or 14.10
Since I couldn't find Caffeine Plus in any PPA, I uploaded it to the main WebUpd8 PPA for Ubuntu 14.04 or 14.10 (if you use Ubuntu 12.04, use the original Caffeine application). Note that by installing Caffeine Plus, the original Caffeine package will be removed automatically.
To add the PPA and install Caffeine Plus in Ubuntu 14.04, use the commands below in a terminal:
If you don't want to add the PPA, grab the Caffeine Plus deb from HERE.
sudo add-apt-repository ppa:nilarimogard/webupd8 sudo apt-get update sudo apt-get install caffeine-plus
Thanks to Willi for the tip!