Network Utility -- Connectivity Checker v2.1
Need to do a quick check of your Web server on the Internet or look up a domain name? This is the utility for you. It comes with tools such as ping, port scan, whois queries and geographic lookup of the IP address range. It is easy to enter your host name (although a ".com" dedicated softkey would save on some keyboard taps). If/when it adds Netcat support, this will be a must-have for security professionals familiar with that tool.
There are numerous other network utilities for the iPhone, such as Traceroute ($2), Host LookUp ($1), iTCPing ($1) and Network Diagnostics Pro ($6), but they have user interfaces that are not as elegantly designed.