0trace.py is a python port of Michal Zalewski's 0trace hop enumeration tool. 0trace was originally announced on Bugtraq on January 6th:
“I'd like to announce the availability of a free security reconnaissance / firewall bypassing tool called 0trace. This tool enables the user to perform hop enumeration (“traceroute”) within an established TCP connection, such as a HTTP or SMTP session. This is opposed to sending stray packets, as traceroute-type tools usually do.
The important benefit of using an established connection and matching TCP packets to send a TTL-based probe is that such traffic is happily allowed through by many stateful firewalls and other defenses without further inspection (since it is related to an entry in the connection table).”