Click here to launch an attack on your machine via email. Click here to flood your network with so many packets that the whole thing goes belly up. Click here to be forever hounded by email from telemarketers. Perhaps you wish to attack someone else's machine. For example, a machine is vulnerable if it is running an old version of NCSA httpd or sendmail or smail or majordomo or in.telnetd or many other buggy pieces of software.
Hackers know of bugs for essentially every kind of machine out there.