Update: Wed 27 Feb 2013, since I'm updating this blog, chrome and firefox and safari all support this feature.
Google Chrome has* one, Firefox has one but Safari does not... until now!
For me, having a manual "load flash only when I tell you to" button in a web browser is a must. This reduces CPU usage on my mac which increases battery life and stops it getting too hot. These are nice things to have!
I understand some people might not want such a feature, but I do so I did some quick googling and came up with https://clicktoflash.com/. It creates a nice smooth looking flash image (shown below) which you click to add the flash item itself.
It also includes a whitelist for sites you know to be ok. Check it out if you run safari on a mac (seeing as windows does not have this problem - its only selling point really!)