How to disable auto-playing Flash and HTML5 videos

"If I had to rate annoying things that I encounter on the Internet on a scale, auto-playing videos would certainly make it to the top of the list, especially if they are not configured to start muted by default. But even if videos start muted, they are a huge annoyance as they may distract users, suck up bandwidth or waste cpu cycles. ..."
Source: gHacks Tech News
---> How to disable auto-playing Flash and HTML5 videos