What's Your Favorite Open Source Software Application?

Recently I read the cover article of PC World which advertised 101 Fabulous Freebies (www.find.pcworld.com/52516 [1]). It is a description of 101 software downloads or Internet based services that are free. A number of the software products listed are open source. Wikipedia [2] defines Open Source software as "computer software available with its source code and under an open source license. Such a license permits anyone to study, change, and improve the software, and to distribute the unmodified or modified software."

Over the the last few months I've gained a much greater appreciation for the variety of applications and incredible value that can be had through the use of applications that are legally available for free. In many of the major categories that I would want to use, there are open source counterparts available. Instead of using Adobe Photoshop, you can use The GIMP. Instead of using Microsoft Office, you can use Open Office. Instead of using Dreamweaver for web design, you can use NVU. After reading the PC World article above, I downloaded PDFCreator, so that I could create PDF files without having to buy another copy of Adobe Acrobat. It works great.

All this made me wonder what applications the readers of this site use and would recommend. Please post your answers as comments below.