tcptraceroute is a traceroute implementation using TCP packets.

The more  traditional traceroute(8)  sends out either  UDP or  ICMP ECHO
packets with a TTL of one,  and increments the TTL until the destination
has  been reached.  By printing  the  gateways that  generate ICMP  time
exceeded  messages along  the  way, it  is able  to  determine the  path
packets are taking to reach the destination.

The problem is  that with the widespread use of  firewalls on the modern
Internet, many of the packets that  traceroute(8) sends out end up being
filtered,  making it  impossible to  completely  trace the  path to  the
destination. However, in many cases, these firewalls will permit inbound
TCP packets to specific ports that hosts sitting behind the firewall are
listening for connections on. By sending  out TCP SYN packets instead of
UDP  or ICMP  ECHO packets,  tcptraceroute is  able to  bypass the  most
common firewall filters.

WWW: http://michael.toren.net/code/tcptraceroute/