In the article The best free open source software for Windows, InfoWorld's Randall C. Kennedy details a list of 10 very useful open source software applications for Microsoft Windows. Of course, the first application mentioned is Mozilla's Firefox, the free web browser that is revolutionizing the Internet. However, as Kennedy points out, there are many more powerful open source applications for Windows just waiting for you to use. Many of these applications fill needs left by Windows and offer a free alternative to equivalent commercial software, and you may notice that I've mentioned some of these applications before.
There's millions of different software applications, but there are only a few applications that nearly everyone needs. A good word processor is something that everyone needs at some point, and the OpenOffice suite offers a competitive alternative to Microsoft Word, as well as the other Microsoft Office product offerings. Image editors come in handy quite often. Paint.net is an easy-to-use image editor, with less power than Photoshop but more simplicity. MediaPlayer Classic is a very powerful and easy to install and use media player. It actually doesn't require any installation at all, and handles more media formats than Windows Media Player and iTunes.
Open source projects offer many powerful, less-common software such as hard drive encryption for securely storing your files. TrueCrypt is one of these applications, providing a superior alternative to the commercial Microsoft BitLocker. VirtualBox is another powerful application, providing an open source alternative to VMware. VirtualBox is perfect for anyone who doesn't need the advanced features of VMware.
You can never be short on Windows utilities, and open source projects offer tons of Windows utilities. If you need to transfer files using FTP servers, then FileZilla is the application for you. I haven't found a better alternative, free or commercial. PDFCreator is a great alternative to the expensive Adobe Acrobat, though it doesn't contain quite all of the same features. Finally, 7-Zip is a superior file compression tool, unmatched by the likes of WinZip and PKZIP.
Kennedy also mentions ClamWin as an open source anti-virus solution, but it doesn't offer the features necessary to truly provide automatic virus protection. However, I agree that the remaining 9 open source applications are definitely worth downloading. There's nothing stopping you, so have at it!