John Gruber of Daring Fireball explains:
ClickToFlash is an open source web content plugin for Mac OS X that blocks all Flash content on web pages by default. As the name implies, if you do want to load a Flash element, just click it. I give ClickToFlash my highest recommendation — everyone should install it.ClickToFlash does for Safari what the Flashblock extension does for Firefox, making webpages less distracting and helping your computer to run at lower temperatures. ClickToFlash is free for Tiger, Leopard, and Snow Leopard.
ClickToFlash (“your web browsing prophylactic”)
[Of the last 500 visitors to Orange Crate Art, 133 — 26.6% — are using OS X.]