BashFlash is a free, low-profile utility that can identify and terminate Flash video processes in Safari, which can help you improve CPU performance and (on a laptop) conserve battery power--and it can even help keep your computer from heating up and running its fans excessively because of Flash overload.
BashFlash only works in 64-bit Safari (i.e., Safari 4 in Snow Leopard), since earlier versions don't run Flash as a standalone process that can be easily terminated. The app sits in your menu bar, and a small BashFlash icon shows your current Flash use (gray for none, black for low, and red for high). When Flash is in use, a drop-down menu gives you the option to "Kill Flash Plug-in," which will stop all current Flash processes, across all windows and tabs, which are then replaced by Safari's "missing plug-in" icon.
Of course, if you're using Flash on purpose--say, if you're watching YouTube--you can leave Flash running. But if your computer is getting sluggish or hot because of too many Flash-based advertisements or some other unwanted Flash process (even running in a hidden tab), BashFlash makes it easy to instantly shut it down. If you want to view the Flash content later, just reload the individual page or tab.
BashFlash is especially useful for laptop users, since it can help save battery life (both short-term and over the limited long-term lifespan of a battery), but it's a worthwhile utility for any Snow Leopard user who gets frustrated with CPU-intensive Flash content.