PingTunnel or ptunnel is an open source software written by Daniel Stodle which can be used to tunnel TCP over ICMP ("ping"). This is called a ICMP tunnel. So with ptunnel it is possible to bypass many forms of Internet cenorship, especially in company networks. It can also be used to bypass the authentication and so the payment for most commercial Wi-Fi Hotspots.
It is intended to run under Linux, but since the latest version it is also possible to run it on Windows. In comparison to other tools which tunnel TCP or SSH over DNS (DNS tunnels) like OzymanDNS, DNS2TCP, NSTX, DaNeSh and DNScat, ptunnel allows way faster surfing speeds of about 150kb/s downstream and about 50kb/s upstream. However it is more commonly blocked on commercial Wi-Fi Hotspots or company networks. Other implementations of a ICMP tunnel include itun, Hans, Skeeve and ICPMTX.
To use ptunnel, you need a Linux server with root access on the "free Internet" where you have to install the ptunnel server and a proxy programm.
The current version is 0.71 from June 22, 2009.
Installing on Debian Linux:
deb http://www.cti.ecp.fr/~beauxir5/debian binary/ deb-src http://www.cti.ecp.fr/~beauxir5/debian source/
to your /etc/apt/sources.list. Then run "apt-get update" and "apt-get install ptunnel".