Free Programs for Windows and Mac

The following applications make excellent alternatives to commercial software and/or default software that may have been pre-installed on your PC.
Most of them also have Mac installs as well.

Video: VLC

Play DVDs and almost all video file formats without the need for installing video codecs.
VLC will take care of most of your video viewing needs with minimal setup required.

Instant Messaging: Pidgin

Easily use all of your Instant Messaging accounts in one console with Pidgin.
Talk to friends on AIM, Google Talk, MSN, Yahoo IM, and more.

Web Browser: Firefox and Chrome

Firefox usually offers a bit more security than Internet Explorer, and Chrome provides much more speed.
Firefox also allows great add-ins to make browsing the internet better.

Anti-Virus: AVG Free, Avira, Microsoft Security Essentials

Basic Anti-Virus protection for your PC.

Archive/Compression: 7zip

I really love this app, because all you have to do is right-click and extract  and zipped file in 2 clicks.  You can create zipped files just as easy.  It even extracts msi files and many zipped file types.

Audio Player: foobar2000

An advanced/high-quality audio player that can be customized as much or as little as you want.

Image Viewer: IrfanView

View basically every type of image file in existence with relative ease. Extensive options make the program highly-configurable.

Office & Word Processing: Google Docs (or Open Office)

Instead of buying and installing software for word processing, spreadsheets and/or slide show presentations, you can just use the web-based Google Docs. Because Google Docs is web-based, you don’t have to worry about losing the files that you’ve created – they’re automatically backed-up for you!

If you’d prefer not to use a web-based application you can check out Open Office instead.