Eclipse, with the newly released PDT 2.0, is a very capable Drupal IDE. With code completion, automatic documentation lookups, and integrated debugging, Eclipse is very good for anyone who spends time doing Drupal code.
One issue I've run into with Eclipse when working on contributed modules is that the modules themselves aren't located within a Drupal installation. I found myself resorting to external programs or the command line to copy my changes to my development site or to my local development copy. Using Eclipse's External Tools, it's possible to deploy changes with a single click, greatly reducing the time to test modified code.
I do most of my website development work in Firefox, due to the availability of extensions such as Firebug, YSlow, and Tamper. Sometimes, I want to check a page in another browser so I can log in as a different user. This simple Applescript, combined with Quicksilver, allow me to easily open the current Firefox page in Safari:
tell application "Minefield" to set theURL to «class curl» of window 1
tell application "Safari"
open location theURL
Update: Unfortunately, as of the September 2009 release, the VPC images from Microsoft have been changed and are now a bit more picky about activation. When you follow this procedure, the images will now fail activation. I've left this page up so that the procedure can be tried on future releases of the images.
Virtualbox is a virtualization product produced by Sun. Available for Windows, Linux, OS X, and Solaris, it not only is fast and full of features, but free! Most of the product is available under the GPL. Some additional features, such as USB and Remote Desktop support, are available free of charge for personal and small-scale commercial use.
Unfortunately, Virtualbox doesn't come with a robust set of tools to convert disk images. While you can convert images between Virtualbox (VDI) and VMWare (VMDK), as well as from straight raw disk images, you can't easily convert images from Microsoft's VirtualPC format. This is an issue when you want to use Microsoft's free VPC images for testing various versions of Internet Explorer. Luckily, with a little bit of help from QEMU, it's possible to convert these images into something usable, all without needing a copy of Windows installed to bootstrap everything.
If you do any website development, and have ever experienced a compatibility issue with Internet Explorer, then this article will be interesting to you. Mozilla is working on getting both the canvas element and Tamarin supported natively in Internet Explorer through ActiveX controls. While the Ars Technica article doubts that many will install such plugins, a few big backers, such as Adobe or Google could make a major change in the web browser landscape. Imagine if the Google Toolbar came bundled with additional controls to fix IE's broken CSS, add support for tags and selectors Microsoft decided not to implement, and speed up Google Maps to boot (by avoiding the ExCanvas library). Exciting work!