As seen on the H Security web site

The Insecure.org developers have announced the release of version 5.20 of Nmap, their popular network scanner and mapper. According to the developers, this first stable update since Nmap 5.00, released last July, includes more than 150 "significant improvements".

In addition to reduced memory consumption and performance improvements, Nmap 5.20 features protocol-specific payloads for more effective UDP scanning and the addition of 31 new Nmap Scripting Engine (NSE) scripts, bringing the total to 80. NSE allows users to create scripts to automate several common network tasks. For better performance, the traceroute engine has been completely rewritten and now sends probes in parallel to individual hosts – previously it would send probes across all hosts. 

The OS and version detection databases have also been updated, bringing the total to 10,000+ signatures. Additionally, the Zenmap host filter mode now only shows hosts that a user is interested in and the Ndiff scan comparison tool and Ncat tool for data transfer have both received updates.

More details about the release can be found in the release announcment and change log. Nmap 5.20 is available to download for Windows, Mac OS X and Linux and documentation is provided. Nmap is released under version 2 of the GNU General Public License (GPLv2) with clarifications and exceptions.