ClicktoFlash

This is a free Safari utility that does just what it says: it takes Flash objects that are embedded in a web page and grays them out unless you click on them. The following is from the website:

Ever wanted to get rid of the scourge of the web that is Adobe Flash, but still retain the ability to view Flash whenever you want? With ClickToFlash, you can! Using ClickToFlash, all of those icky Flash bits that have infected most webpages on the internets are replaced with a nice, smooth gradient and the word "Flash" set in a nice, pleasing font. When you want to view the Flash, just click on it!

The advantages of ClickToFlash are numerous. Since Flash isn't loaded until you specifically ask for it, your CPU usage will stay at normal levels when browsing the web. This has tons of benefits: web browsing stays speedy, your Mac laptop won't get as hot, and your Mac's fan won't come on as often. In fact, we guarantee* that ClickToFlash will quintuple your battery life and that it will protect those precious parts of your body on which you rest your laptop! (*note: not actually guaranteed)

This is what a Flash grayed-out box looks like:

You double-click in the box to launch the Flash file (YouTube videos are Flash files). The little gear in the upper left-hand corner has other options and settings.

I've loaded this utility and it works just as described.  Find it here.