I use many of these plugins on a daily basis and would recommend them to any web developer.

1. Firebug

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

Firebug has a number of extensions that will be covered later on in this article.

2. Web Developer Toolbar

The Web Developer toolbar has various web developer tools. The most useful I find are for disabling certain elements on the screen such as JavaScript or clearing cookies and session data.

 Web Developer

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.

3. Extended Statusbar

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.

 Extended Statusbar

4. Colorzilla

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.

 Colorzilla

(Note US spelling 'color').

5. MeasureIT

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.

 MeasureIT

6. Screengrab

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.

 Screen Grab

7. YSlow

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.

 Yslow

8. Page Speed

Page speed is similar to YSlow however covers slightly different criteria. As you can see for ImAFish it highly recommends combing external JavaScript.

 

9. Linkification

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.

 Linkification

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.