Appstorm has a great article listing the top 20 free Windows apps as of late 2011. I strongly agree with some of them but all are worth looking at depending on your needs.
I especially recommend Microsoft Security Essentials, Chrome, Paint.NET, Wunderlist, LastPass, XMarks, 7-Zip, VLC, and LibreOffice as useful for everyone. Others such as Handbrake are useful depending on how you use your PC. Many of these are open source as well as free.