The best "free" software that I use

My friend James asks, What is the best 'free' software you have stumbled upon?.

I seldom stumble upon free software, I usually go looking for it. My home computer runs Windows XP Home Edition, so I'm always looking for free software that improves upon it. I will say that the most important piece of free software on my PC is what I'm using right now: Mozilla Firefox. If you're still using Internet Explorer, I pity you.

I'll skip the usual free software that everyone has (Windows Media Player, iTunes, Adobe Reader, etc.) and talk about the free software that you might now know about:

• Google Desktop: I've turned off
  the sidebar and all its distracting gadgets and pop ups and use this
  strictly to search my computer. Since I have emails going back to 1998,
  this has been a lifesaver.
• Notepad++ and Vim: Windows ships with Notepad, which is probably the most pathetic and useless text editor. If you edit text files regularly, like I do, you absolutely need a replacement text editor. I use Notepad++ for most of my text file viewing and editing needs and it's all I need much of the time. However, if I need to do some complex search and replace using regular expressions, vim makes it easy. Easy for me, that is -- if you've never used vim or vi and you're not up for a challenge, I can't recommend it to you.
• PDFCreator: If you're on a Mac, you don't need this. Macs can create PDF files without any additional software. If you have Adobe Acrobat, you probably don't need this either. But if you don't have either and you don't want to spent hundreds of dollars to create PDF files, PDFCreator allows you to create them from nearly any Windows application. Plus, it's open source and ad/nag-free, unlike PDF995, which is what I used to use.
• WinMerge: This handy tool is a file comparison utility that can compare two files or two directories.
• FileZilla: This is a free open source ftp client that I use whenever I need to connect to an ftp server.