As a web developer, I have a web browser running about 99.9% of the time I'm on a computer. It's usually the first program I start up and the last one I shut down. Not only is a web browser the medium through which the world views our work as developers, it can also be an indispensable tool to help us get sites built. Most desktop browsers have a few tools built in that can assist in web development, but the real power comes when you get into the realm of browser extensions.
My personal choice of web browser happens to be Firefox, which puts me in the minority these days. Google's Chrome browser currently has a commanding lead in browser market share. Probably the only reason I now use Firefox is that I've been using it since before Chrome was a major player and before Chrome had many of the extensions that I find so useful in my daily work. I should note though, that that time has long passed. Many of the extensions I use are now available for both Firefox and Chrome so it's really only habit that keeps me on Firefox. Maybe I'll switch one of these days...
But enough about Firebug. Two more extensions that I always download on a fresh browser install are ColorZilla and MeasureIt. In contrast to Firebug, these extensions are basically single-function add-ons. The former provides a color-picker that lets you sample the color at any point on a web page, and the latter a screen ruler that lets you measure. Both are very useful when doing template work and, incidentally, both are available for Chrome as well as Firefox. Once you pick a color with ColorZilla, the hex code for that color is copied to the clipboard in standard css notation, but the dropdown menu it provides also allows the color to be copied in a variety of other formats, so it can be useful in conjunction with other graphics programs as well as for add colors to a style sheet.
A final extension that I only recently discovered but which is quickly earning a spot in my favorites list is S3 Organizer a.k.a. S3Fox. It's just a simple two-pane style file transfer tool that shows your local files on the left and the remote files on the right. I came across this when I was searching for an S3 client to use on my Ubuntu development machine and it has worked great for me from the start. It's nice to have this accessible in my browser with just a couple of clicks since my browser's already open.
Of course this list only scratches the surface of what is available, but these are the extensions I find myself using over and over again. There are many more useful extensions that I use from time to time, but not necessarily on a regular basis as I do with these. Not to mention there's a whole other list of extensions that I use as a consumer of web sites rather than a developer. What browser extensions do you find help you get your job done whether you're a developer or otherwise?