Listed below are freeware alternatives to popular software packages.  All are free for home/non-commercial use and are great products!

avast! Anti-virus software with several configurable modules (standard, Outlook, instant messaging, web, etc.).

Spybot, SUPERAntiSpyware, Malwarebytes’ Anti-Malware Anti-spyware software.  Make sure you update regularly!

Comodo Firewall Firewall which monitors both incoming and outgoing traffic.

Paint.net Graphics and photo editor

IrfanView Image viewer plus plus plus!  Very robust featureset (batch file conversion, standalone EXE slideshow creator, etc.).

JAlbum Create HTML-based photo albums with ease.  Several templates included.

Audacity Audio editor

Foxit Reader PDF viewer

CutePDF Create PDF files with ease!

LibreOffice Alternative to Microsoft Office and Corel WordPerfect Office.

ImgBurn CD/DVD burning software.

Belarc Advisor Get a hardware and software inventory of your computer.

Camstudio Capture whatever you do onscreen and save it in AVI or Flash format.  Great for training videos, presentations, etc.