sbd : Backtrack 5: Maintaining Access: OS Backdoors: sbd
The sbd command is a netcat clone that offers decent encryption, allows for command execution, and will install on Linux or Windows. You are likely aware of netcat and all of the different ways you can use it including reverse shells, port scanning, etc. The sbd command is a great tool for opening up protected listeners or as noted in the Backtrack Linux menu for maintaining access to servers on compromised networks. Since it is likely you are familiar with nc or netcat below I go into setting up a reverse shell using a secret key between the listener and the connecting client.