Sub7 or Subseven (also known as Backdoor-G and all of its variants) is the most well known Trojan backdoor application available. You can download the setup-file of Sub7 from anywhere.

The first version from SubSeven appeared in May 1999. SubSeven 2,0 is present since September 1999. Apart from an extension of the characteristics above all the configuration options for the server installation were extended. Beside the Client and the server program, also again the Tool is contained for the modification of the actual server "EditServer" in the Orginal Zip file. Download an infected email attachment could be worse!
This trojan is the most popular and the most powerful Trojan Horse program available to the public