Open source I use and recommend

I'm always happy to put in a plug for the tools I use every day. With the exception of the Anti-Spyware category, all of the following software is not only free, but open source as well. You might have heard of, or even use, most of these programs already, but give the list a scan for any hidden gems you might not have encountered before.

Virtual Desktop
Organize your applications over several virtual Windows desktops. In this "once you've tried it, you can't live without it" category, we have two strong contenders: VirtuaWin and Virtual Dimension. The latter application has some extra features, but it's a virtual (sorry) tie: you can't lose trying either one of them. [Windows Only]

File Archiver
7-Zip can create ZIP, GZIP, BZIP2 and TAR archives, and can unpack RAR, CAB, even your old LZH and ARJ files. And the Windows version integrates into Windows Explorer. What's not to like?

Server Operating System
I am always happy to put in a word for my favourite Unix server operating system, FreeBSD. While GNU/Linux may have better support for bleeding-edge hardware, I find FreeBSD more stable, scalable, secure and usable. When you consider all the various Linux distributions out there, the fact that there is one FreeBSD is, frankly, less confusing. The excellent ports system and the fact that FreeBSD can run Linux binaries are icing on the cake.

X Server
If you need to run a particular X application on Windows, or if you prefer xterm to putty, you need Cygwin/X. It's a large download, and there are decent commercial alternatives in this category, but this is my pick. [Windows Only]

Web Browser and Email
Yes, Firefox is slowly but surely winning the browser war, so I don't need to plug it a whole lot. Thunderbird will probably do the same thing to Outlook eventually, once more people try this full-featured (spell-checking, address book, spam filter, etc.) email client that provides the same slick email interface whether you're running Linux, Windows or OS X. Highly recommended.

Database
Which one is best: MySQL or PostgreSQL? The debate rages on, but the two have recently come much closer in terms of capabilities, with MySQL playing catch-up in terms of features (stored procedures, triggers, views, etc.). It's getting tougher and tougher to pick a winner, but I think they each have their strengths and appropriate place, and quite honestly, I'm as happy as a clam using either one.

SFTP Client
If you need to securely transfer files between a Windows client and a remote computer running SSH, and you want a GUI interface, FileZilla will make your life easier. [Windows Only]

Text Editor
If you know what a text editor is, you probably already have a favourite, and if past experience is any guide, you probably will use it exclusively the rest of your days. The one they will pry from my cold, dead fingers is Vim.

Terminal Emulator
If you work on Windows, but need to access remote Unix accounts, how can you not already love PuTTY? If only they would finish the Mac OS X port, I could die happy. Until that day, I'll use iTerm on OS X, if only for the tabbed windows. In conjunction with either of these terminal emulators, I could not work without GNU Screen, a console window manager that allows you to run interactive shells in multiple virtual terminals.