httptunnel was created by Lars Brinkhoff (http://www.nocrew.org/software/httptunnel.html). From the README:

httptunnel creates a bidirectional virtual data path tunnelled in HTTP requests. The requests can be sent via an HTTP proxy if so desired.

This can be useful for users behind restrictive firewalls. If WWW access is allowed through an HTTP proxy, it's possible to use httptunnel and, say, telnet or PPP to connect to a computer outside the firewall.

This SonicWALL signature identifies httptunnel encoded traffic over HTTP Proxies.