How can I stop Safari from playing videos on a site automatically?
The Safari extension ClickToPlugin is similar to other plugin-blocking extensions, but it has many features and customizable settings which most blockers don't, including the ability to block plugins other than Flash. Scroll down the linked page to see some of the other features.

Here is a screenshot of an example YouTube video, showing the video preview along with the type of plugin blocked:



Here is a sample of some of the settings in ClickToPlugin: