Subterfuge is a python based tool that you can use for testing Man-In-The-Middle attacks. the program will start to sniff network traffic and wait to have user login to online websites such as gmail , twitter, facebook and more. Next it will display accounts information.
Beside the session hijacking module there are other modules that can be used for scanning the network, perform an HTTP code injection or Denial of service. there is also a Tunnel Block to prevent certain protocols from accessing the internet including PPTP, Cisco IPSec, L2TP, OpenVPN, SSH.
Another module is the Network View where it provides a rapid sync to represent victim information in real-time. another addition is Evilgrade which allow attacker to spoof operating system update and provide victim patches shipped with backdoor.
You can read more about Subterfuge on the following link: http://code.google.com/p/subterfuge/