Written by Fyodor of insecure.org, nmap is one of the most common and most popularly noted tools in any hackers arsenal.
It's common abilities are port scanning host on a network to determine which ports are open and which services are running.
It has a very wide feature set of determining what device is being scanned whether it's cisco, juniper, windows98, windows 2003 server, red hat, debian, suse/novell, etc...

It does support 64bit processing now, it does support ipv4 & v6, it does support tcp & udp, it does support mac, linux, bsd, solaris, windows, and a whole bunch of other operating systems that you may pray to never have to work with.
From personal experiences it runs on my ps3, so just about anything out there.

Nmap has the ability to not only obtain information about a host by querying that host, but can also obtain information about a host by querying information for it, from it's peers on the network.
By using Nmap's TCP Idle Scanning technique it has been deemed possible to bypass filters such as access list, ip tables, and intrusion prevention systems under certain circumstances.

For more information about about Nmap please go to http://nmap.org/
For more information regarding the TCP Idle scan the link is http://nmap.org/book/idlescan.html