Appcap is a tricky application for x86 Linux which allows an user with enough power (usually the superuser) on a machine to attach and redirect standard input and output of any application to his actual tty.
Appcap can help you to trace users, for example if you are running a multiuser machine and are suspicious that some of them may be doing nasty things using your machine. It is especially very useful for tracing and monitoring ssh and telnet sessions.
