The anti-virus or anti-malware test file. A number of anti-virus researchers have worked together to produce a file that their (and many other) products "detect" as if it were a virus. It is safe to pass around, because it is not a virus, and does not include any fragments of viral code. Most products react to it as if it were a virus (though they typically report it with an obvious name, such as "EICAR-AV-Test").....(free).....