So you go to visit a website and a video starts playing before you can even say "hello." Ad blockers such as AdBlock block auto-playing video ads, but what about videos that aren't ads? There's a way to stop them, too.
Here's an article from HowToGeek.com on how to stop auto-playing HTML5 videos in Chrome, Opera, and Firefox.
Although Adobe Flash is on its way out, there are still plenty of websites that use it. Here's an article from PCWorld.com on how to stop auto-playing Flash videos in Chrome, Firefox, and Internet Explorer. It's a bit dated now (the screenshots show older version of the browsers), but the general procedure remains the same.
If you use Safari, here's a great article from the Nerdily blog on how to stop auto-playing HTML5 videos in Safari. Here's how to stop auto-playing Flash videos in Safari, courtesy of DFWCI.com. (Note: Safari will include a way to block auto-playing videos natively with the High Sierra update to MacOS.)
Where is Microsoft Edge? There is apparently no way (yet) to stop videos from auto-playing in Edge. As Edge's extension support becomes more robust and more extensions are ported over from Chrome, this may change. We'll update this article as we learn more!
Bonus tip: Browse quietly
What if you just want a little peace and quiet while you browse for cat videos...we mean, work? Here's how to turn off unwanted sound on web pages in Chrome and Firefox, courtesy of ZDNet. It even includes tips for disabling auto-playing videos.
Update August 5, 2017:
Apparently the Chrome extension Disable HTML5 Autoplay is no longer being maintained by the original developer. You can probably still use it, however.
Update August 8, 2017:
Chrome 61 (currently in development) includes a flag to disable auto-play. Finally, hope is on the way!