ClickToFlash is a great Safari plugin that blocks Flash from automatically loading and playing.
All it takes is a single click to play the Flash. This will cause your pages to load faster, and crash less, if you are one of the many that believe the Flash plugin crashes often.
This wonderful plugin goes even farther, though. If you are on youtube, you can use the gear icon in the upper left of the Flash window to either play H.264 version of the video in QuickTime (without all of that Flash crap on the screen) or download the video directly.
Very nice.