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.
Ubuntu makes many things easy. For example, with Gnome, it's very simple to set up remote access to your desktop with the included Vino application. The only problem is that to get the VNC server running, you have to be logged in! Luckily, it's not too difficult to create a script which starts up the server as soon as your system boots. This is especially handy if you are running a headless server, or are using the primary display for other purposes.
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!