I'd like to thank Steven Frank for posting the link to ClickToFlash ClickToFlash, a Safari plugin that prevents the loading of Flash content until you click it.

I shared this plugin's location with John Gruber and he noted that he was seeing low single-percentage CPU usage with 30 Safari tabs open. In my own tests, Safari took up 4% CPU when looking at a YouTube page prior to loading the Flash video. After loading it (by clicking the grey gradient that replaces the video), CPU usage rose to 40 to 50%.

It works, and it's pretty great.

Update (2009-01-28): Updated link to the current fork.