CSS Mania - Showcases amazing CSS designs. Updated daily.

CSS Tricks - Great tutorial/blog site covering CSS, jQuery and even some PHP.

Smashing Magazine - Web Design community.

Microsoft's ASP.NET Website/Community - One-stop spot for all that is Microsoft on the web.

General

ClipX - Great tool for keeping a history of things you've copied to the clipboard (text and images) adding the ability to quickly access that history with shortcut keys.

IIS Admin - Ignore if you use Windows 7. Allows for multiple websites in IIS in XP.

Fiddler - Web Debugging Proxy - This tool has saved my life dozens of times. Great for debugging web/service requests/responses.

jQuery - Write Less, Do More - If this JavaScript framework was a person, I would marry it.

JSFIDDLE - Great online tool for testing out JavaScript, HTML and CSS with various JavaScript frameworks (jQuery, most importantly)

Launchy - Indexes your start/programs menu (and whatever else you want) and with a couple keystrokes you can quickly search and run what you're looking for. I never open my start menu thanks to this program.

Microsoft Web Platform Installer 2.0 - Program for installing and updating various frameworks, server/database tools and popular applications (Wordpress, Joomla, etc.)

Process Monitor v2.93 - Monitoring system that allows you to see real-time file system, registry and process/thread activity. Great for debugging.

soapUI - Pretty cool tool (despite being Java-based) for testing/debugging web services.

Visual jQuery - A jQuery-Driven API for the jQuery Library - API reference for the jQuery framework.

WinDirStat - Running low on disk space? This is a neat little tool that will tell you what directories are taking up the most space.

Firefox

If you're a web developer, chances are you're using Chrome or Firefox for most of your development because they both sport some pretty snazzy plugins/tools for web development.

Although Google's Chrome browser is picking up in popularity and has some decent plugins, I'm still a fan of Firefox so I primarily use it for development. What about IE? Well...we'll see how v9 goes. :)

Delicious Bookmarks - If you use Delicious to keep track of sites you liked, then this is a handy tool to have. Logs into your account and lets you access your Delicious bookmarks from a side bar.

Firebug - I use this to inspect HTML, CSS, web requests & responses--which is awesome for figuring-out what's going on with all of your AJAX requests--and to debug JavaScript.

Firecookie - An extension for Firebug that lets you view and manipulate your browser's cookies.

Greasemonkey - Lets you run JavaScript against any site you want. Great for tweaking sites you frequent to get rid of annoying ads or even restructure DOM elements with jQuery.

Screengrab - Lets you take a snapshot of a portion of the browser window or everything in the window (including content out of the screen). Great for doing screenshots for clients.

Web Developer Toolbar - A must tool for any developer.