I am curious what the opinion is of AO members about what the best open-source and / or freeware tools are for the various categories of security software. I know that Fyodor has a Top 75 list on the NMap site, but I want to get a sampling of AO opinion.

What do you think is the best open-source or freeware tool for:

Intrusion Detection
Vulnerability Scanner
Port Scanning
Packet Sniffing
Spam Blocking
Spyware / Adware Blocking or Removal
Enumeration Tools
Other Miscellaneous Tools

It would be nice if you could also share WHY you feel that way- what about this product makes you nominate it for "best of class"?