twit88.com

WebMail Notifier checks your webmail accounts and notifies the number of unread emails.

Currently supports

mail.google.com(Gmail & Google Apps)
mail.yahoo.com
mail.live.com(hotmail,msn,live)
www.daum.net(hanmail.net)
www.naver.com
www.empas.com
www.nate.com

Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them. Right-click on a banner and choose “Adblock” from the context menu — the banner won’t be downloaded again. Maybe even replace parts […]

FoxyProxy is a set of proxy management tools for Firefox, Thunderbird, Seamonkey, and Songbird.

Animated statusbar/toolbar icons show you when a proxy is in use
Define multiple proxies
Define which proxy to use (or none!) for arbitrary URLs using wildcards, regular expressions, whitelists, blacklists, and other conveniences
No more wondering whether a URL loaded through a […]

TamperData is an extension to track and modify http/https requests.
What I use it for

security testing of Web based applications
tracking request/responses
It’s just cool.

ColorZilla is an extension for Mozilla Firefox and the Mozilla Suite.
It assists web developers and graphic designers with color related tasks - both basic and advanced.
With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you […]

lori can tell you how long it takes to load a page. Useful for web developers who are working on profiling, optimizing a web site.

Vimperator is a free browser add-on for Firefox, which makes it look and behave like the Vim text editor. It has similar key bindings, and you could call it a modal web browser, as key bindings differ according to which mode you are in.

Vim-like keybindings (h,j,k,l,gg,G,0,$,ZZ,<C-f>,etc.)
Ex commands (:quit, :open www.foo.com, …)
Tab completion available for […]

YSlow is a Firefox add-on integrated with the popular Firebug web development tool. It analyzes web pages and tells you why they’re slow based on the rules for high performance web sites.
It features

Performance report card
HTTP/HTML summary
List of components in the page
Tools including JSLint

FFClickOnce is a Firefox extension that adds an extra option when you click on a link to a ClickOnce application.
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ClickOnce is a deployment technology that allows you to create self-updating Windows-based applications that can be installed and run with minimal user interaction.
A ClickOnce application is any Windows Forms or console application published using ClickOnce […]

SwitchProxy Tool  is a Mozilla tool you can use to organize all your proxies. You can add, edit and remove them from a single master list. When you need to use one, just select it from the list and click apply.

It also has an Anonymous Proxy feature. An anonymizer uses a list of proxies […]

Aardvark is a Firefox extension that can be used to

Clean up unwanted banners and surrounding “fluff,” especially prior to printing a page
See how the page is created, block by block
View the source code of one or more elements

The Pencil Project provides a free and open source tool for making diagrams and GUI prototyping.

As quoted from the website, it features

Built-in stencils for diagraming and prototyping
Multi-page document with background page
On-screen text editing with rich-text supports
PNG rasterizing
Undo/redo supports
Installing user-defined stencils
Standard drawing operations: aligning, z-ordering, scaling, rotating…
Cross-platforms
Adding external objects

It is released under GPL version […]

One thing I like about Firefox is the abundance of extensions out there which can do almost anything that you can think of.
Solvent is a Firefox extension that helps you write screen scrapers for Piggy Bank.
Piggy Bank is a Firefox extension that turns your browser into a mashup platform, by allowing you to extract […]