To keep videos and other Flash content from automatically playing when you open a web page, type 'chrome://settings/content' into Chrome's Omnibox, scroll down to the section labeled 'Plug-ins' and select 'Click to play'. Now, any piece of multimedia content will appear as a grayed-out box until you click to activate it.
Be aware that this may cause issues with certain websites. You'll probably want to click the 'Manage exceptions' button and white-list sites that rely heavily on plug-ins to operate (YouTube, Vimeo, Pandora, etc.) in order to avoid any funny behavior.
