Some links to quality and popular Open Source software

What application do you want?

Anti virus? Try ClamAV for unix/linux or ClamXAV for Mac OS-X or ClamWin for Windows.

Word processor, Spreadsheet, Presentation etc.? Try OpenOffice

Web browser? Try Firefox, Webkit, Chrome (from Google), Safari (from apple - based on WebKit), Stainless

Mozilla Firefox runs on Windows, Mac OS-X, and Linux PCs
Webkit runs runs on (Windows and Mac OS-X PCs. Safari probably a better version for most people.
Google Chrome runs on just Windows for now. You can get an beta Mac OS-X version here.
Apple Safari runs on Windows and Mac OS-X PCs
Stainless (closed source?) runs on Mac OS-X. It is tiny, very fast and very simple. I added this because it is an interesting project.

Graphics editor? Try the GIMP

Media player? Try VLC

Bit Torrent client? Try Transmission (open source).

Sound editor? Try Audacity

Email client? Try Mozilla Thunderbird.

Virtual PC environment? Try Virtual Box

This allows you to run other operating systems (guests) on your current OS (host). For example you might like to run Linux on your Windows PC. Linux would run in a window or full-screen if you like and at the same time you can run all your other Windows applications.

Virtual Box allows you to easily install multiple copies of Linux and even other Windows versions on your current Windows PC. It also works on your Mac so you can run Windows and Linux on your Mac - this is what I do. You can start them up and shut them down just like real PCs.