Adobe Reader
http://www.adobe.com/
Windows | MacOS | Linux | Mobile This simplest of Adobe's PDF programs lets you do just about anything PDF-related (besides create new ones), including online collaboration. It includes a host of features to aid users with disabilities.

AIM
http://www.aim.com/
Windows | MacOS | Linux | Web One of the most widely used pieces of free software ever, AOL Instant Messenger offers a ton of capabilities.

Audacity
audacity.sourceforge.net
Windows | MacOS | Linux Whether you're recording or editing, Audacity is all about audio in practically any format.

Firefox
http://www.mozilla.com/
Windows | MacOS | Linux This PC Mag Editors' Choice Web browser has been on top of the heap since version 1.5 came out in late 2005

GIMP
http://www.gimp.org/   
Windows | MacOS | Linux The GNU Image Manipulation Program (GIMP) does most of what Photoshop does; the Gimpshop project (plasticbugs.com) even makes it look like Photoshop. 

iTunes
www.apple.com/itunes
Windows | MacOSWhen you're attached to the top media player in the land (iPod), success is a given. iTunes continues to build sales and refine its organization of songs, video, games, podcasts, and more. 

OpenOffice.org 
http://www.openoffice.org/
Windows | MacOS | Linux You can spend a lot for Microsoft Office or nothing for this suite with full-function word processor, spreadsheet,  database, presentations, even an equations editor.  We now ship this suite of programs as our ‘office software of choice’. It has some cool features you don’t find in MS Office!

Skype
http://www.skype.com/
Windows | MacOS | Linux You'll pay to call regular phones, but if you sign up all your friends, Skype provides easy (and even international) calls and video-conferencing for nothing.  We use it for international conferences with our POS software developer.

Thunderbird
www.mozilla.com/thunderbird
Windows | MacOS | Linux Mozilla's no-cost e-mail alternative is extensible, fast, and easy to master. And a wealth of free add-ons means there's not much this program won't do, from calendars to encryption. 

AVG Anti-Virus Free Edition 
free.grisoft.com
Windows Its control ce