Caffeine

A simple extension to keep your operating system from going into hibernate or sleep mode which works at two levels:

"display": only prevents all your displays from dimming or displaying screensavers
"system" prevents the entire system from going into the power-saving mode

This extension only offers a single toolbar button which is inactive by default. You can click the button once to active the keep awake state and another click will release this state. To change the level, use the right-click context menu items.