If you're installing a critical security update on your computer, caused by the software vendor's sloppy code quality, you probably wouldn't dream that your software vendor is trying to make some money out of the inconvenience.
CNET offers a download of the free Nmap software, but not from Nmap's own site. Instead, CNET has wrapped the Nmap installer with a program of its own. On your way to Nmap, you're pushily offered a toolbar first.
Find out why this sort of thing is a Bad Idea.