Shouldn't live without anyway. Here's a rundown of some of my personal favorite tools for web development. All software and plugins i mention and link to in this posting are open source / free unless otherwise specified.

Many of these are dependent upon Firefox, if you don't have firefox get it here:

(The google toolbar adds the ability to instantly see page rank of any site you're looking at.)

Diagnostic plugins for Firefox

1. Firebug: a must have tool with html / css inspection, view source of dynamic / ajaxed elements, real time editing of the view layer of any page you're browsing and much more
2. YSlow plugin for Firefox: a plugin for Firebug that analyzes page load / render times and 'grades' elements of page rendering, offering suggestions for improved performance.
3. LiveHTTPHeaders: this Firefox plugin shows you every request that your page makes. Very helpful for seeing the whole picture of what's going on behind the scenes.
4. Mouse-over css inspector: a simpler way to view the CSS properties on page. (simpler, more single-purposed than Firebug)
5. A very comprehensive list of other Firefox plugins

Text Editors:
Since I'm PC based i'll be primarily addressing PC text editors. However, i know that TextWrangler is a decent, free editor for Mac, and if you're willing to spend ~40 bucks, you can pickup TextMate, the wildly popular mac editor.

For PC, there are a number of excellent, free editors out there. For a long time i had been using the not-free "Home Site" from Macromedia, but they discontinued the product, and I've found some of the free products out there to offer very similar functionality.

Everyone has their personal favorites. Many use Eclipse, jEdit is also popular. Eclipse has never been a favorite of mine, and jEdit is java based, so it has to fire up the whole java environment, and loading and use seems slow and cumbersome to me.

My *current* favorites are PSPad and Notepad++

Both offer built in file browsers, multi window / split screen views, user customizable highlighter settings, a number of user-contributed add-ons, handling for most popular languages such as HTML/CSS (obviously), PHP, Ruby, asp, etc.

PSPad adds an integrated FTP client, file diffing, and slightly more robust menu and configuration options. It's not quite as lightweight as Notepad++, but pretty close.

Personally i use PSPad for coding up PHP/ROR/HTML, and now use Notepad++ solely for editing stylesheets. I like to have two physically separate windows / programs open simultaneously, PSPad for coding, Notepad++ for CSS.

Again, there are dozens of solid open-source development tools out there, but those are my picks, and I hope they offer at least a good starting point to anyone new to the world of development