netcat could be considered a trojan i guess, as stated in Lithium's post, but it can be used for many other legitimate purposes as well. I use it mainly for port redirection for outbound traffic from a secured subnet. DNS, SNMP and other services could all point to the netcat host, and the netcat host then redirects traffic through the firewall. The benefit to this of course is that you only have to allow outbound traffic from a single source IP on the firewall.