Web Developer

If you're a web developer and you use Mozilla, let me recommend to you Chris Pederick's Web Developer toolbar. Basically, it puts a toolbar (surprise!) with a whole-load of nifty features, like "validate my HTML code" or "Outline images with ALT attributes" or "Resize to 640x480".

My personal current favourite is "View ID and Class details" which shows you which bits of your webpage belong to which CSS ID's and Classes. Very cool.