I use many of these plugins on a daily basis and would recommend them to any web developer.
Firebug allows you to inspect elements on a web page and see the associated attributes. You can edit in real time allowing you to quickly make changes to an elements colour, position, border or behavior.
Firebug has a number of extensions that will be covered later on in this article.
Clicking display alt tags under the images menu is a quick way to see if you have missed any. The resize menu lets you test your site in different resolutions.
Extended statusbar is useful to way to see how large a page is to download and how long it takes to load. I often use this to help monitor slow loading pages.
Colorzila allows you to mouse over elements on the screen to see the colour. Its very useful if you need to match a colour or check a colour is correct.
(Note US spelling 'color').
MeasureIT simply allows you to measure elements on a screen and gives you the measurement in pixels. Very useful for checking the height of objects, text spacing and the overall width of the page.
An easy way to take screen shots of a page. Especially useful if the page includes scroll bars as it saves you copying and pasting multiple screen shots together.
YSlow shows you various options on how to speed up your website. Clearly not all are suitable/cost effective for a website such as ImAFish however many are very relevant. Each item is given a rating from A to F and a full explanation on how to implement. YSlow is developed by Yahoo.
8. Page Speed
Linkification is a simply little plugin that converts any URL on the screen into a clickable link. Quite useful for those times someone has forgotten to turn a URL into a link.
10. IE Tab
IE Tab is a nice plugin to test your page in Internet Explorer without actually having to open the Browser. It's also useful if you want a fresh session for a website without having to open another browser.