DownloadHelper - This helps you copy most kinds of media on a web page (I mostly use it for embedded Flash videos (.flv))
NoScript - This is my most useful add-on... but it can be quite annoying to certain people
FlashBlock - Also very useful... but can annoy some people
SwitchProxyTool - To switch proxy servers quickly... probably of little use to most people
GreaseMonkey - Something I use to screw with certain sites
