ClickToFlash is a free plug-in for Safari that can--in the words of the developers--"block evil Adobe Flash...the scourge of the Web." Indeed, ClickToFlash does an excellent job of blocking unwanted Flash video while still letting you watch Flash when you want to, which can result in better browser performance, fewer crashes, cooler running temperatures (and hence lass fan noise), and in the case of laptops, longer battery usage (in both the short term and over the life of a battery).
ClickToFlash is easy to install and use. Once installed, ClickToFlash blocks all Flash video, instead showing a gray box with the word "Flash" and small settings icon in the upper left corner wherever Flash would otherwise appear. You can then load the Flash video by just clicking the word "Flash," or you can access the settings and instruct ClickToFlash to load all Flash on the page or to add the video's source URL to a "whitelist" (so, for example, if you don't want ClickToFlash to ever block video from a site like YouTube or ESPN, you can just add that site to your whitelist and those videos will always load normally).
This functionality alone makes ClickToFlash worth installing, but this app adds a couple extras, too: you can set ClickToFlash to always load higher-quality H.264 videos from YouTube (the format used for YouTube on the iPhone), or you can even set it to automatically load HD video when available.
If you're ever annoyed by Flash videos while surfing the Web (especially processor-intensive ads), ClickToFlash will be a welcome addition to your browsing experience. If you're on a MacBook, ClickToFlash can also add to your effective battery life, making it definitely worth checking out.