nmap has got to be one of my favourite and oft used tools.
I am using it as I type this   :)

From nmap-hackers:
Hot on the heels of 3.9999 (you could probably guess this was
coming), I am pleased to announce that Nmap 4.00 is now available!

Documentation: http://www.insecure.org/nmap/docs.html
Download: http://www.insecure.org/nmap/download.html
Release Announcement:
http://www.insecure.org/stf/Nmap-4.00-Release.html

Cheers,
Adrien