Flash is ubiquitous on the web today. This is not necessarily a bad thing, as it can do awesome things like an interactive slide show about the Hudson river crash of US Airways Flight 1549. However, Flash interferes with my web browsing habits: I almost always open links in new tabs, so I can keep reading the current page while the other pages load (CTRL-click or Command-click). However, if many pages have Flash objects, such as ads, my computer slows down. Worse yet, this consumes laptop battery power for nothing. The solution: only load Flash objects when you want, by clicking on them. For Firefox, install the FlashBlock extension. For Safari, you want clicktoflash, which was recently released.
One disadvantage of blocking Flash is that there are some sites that use Flash behind the scenes, without a visible Flash spot for you to click on. In these cases, you need to whitelist the site to get it to work correctly. To whitelist a site with clicktoflash, use the following command in Terminal: defaults write com.apple.Safari ClickToFlash.whitelist -array-add 'site name e.g. nytimes.com'