Nmap 5.00 has been released! This version adds some interesting capabilities
to an already-impressive open source project. Among these changes are the
addition of Ncat and Ndiff tools, dramatically improved performance, and a
number of improvements to scripts in the Nmap Scripting Engine.
Documentation has been expanded to cover these new features.

Here's a quick list of the 5 most significant changes:
http://nmap.org/5/#5changes
Another list with some further details: http://nmap.org/5/#changes

Download page: http://nmap.org/download.html