Firewalk - a technique developed by MDS and DHG that employs traceroute-like techniques to analyze IP packet responses to determine gateway ACL filters and map networks.
Hping - a complex ping-based program that allows you to send customized pings to remote hosts and networks, gathering raw fingerprint data, utilizing the concept of "tcp specific firewalks," and more.
Nmap - a utility for network exploration or security auditing. It supports ping scanning (determine which hosts are up), many port scanning techniques (determine what services the hosts are offering), and TCP/IP fingerprinting (remote host operating system identification). Nmap also offers flexible target and port specification, decoy scanning, determination of TCP sequence predictability characteristics, reverse-identd scanning, and more. The latest version of netmap comes with a GTK+ front-end called nmapfe and there is a KDE gui called Kmap. Is it just me, or is KDE really ugly??
More Tools
DNS Tools - bunch of useful
DNS querying tools by Paul Balyoz
Dnswalk - Perl script for debugging DNS databases
Exscan - scanner that capture banner info and uses Queso for OS detection
MTR - diagnostic tool that combines the features of traceroute and ping
Nlog - a set of perl scripts that allow you to search through your Nmap 2.x scan logs. Included is a conversion script and a complete CGI interface with 4 extensions and support for more. From your web browser you can search for all hosts with any given port open, operating system, sequence index, or IP address and query common services through the extension scripts.
TDS - Top Level Domain Scanner- a script written in perl for unix which utilizes data from dns information, essentially scanning the entire internet by querying the root-servers, and then the individual nameservers for domains
Queso - OS fingerprinting tool by the Apostols (site is down)