Earlier quoted context omitted.
Another common one is the EICAR anti-virus test file. X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H* It is a com executable that prints EICAR-STANDARD-ANTIVIRUS-TEST-FILE! It recognized by most virus scanners.
Oh, cool! I thought this file was just prefixed with some random garbage. Unrelated: I used this to verify that my daily scheduled full scan of my Linux laptop works. This is required by compliance at $WORK. It reports found viruses via the i3-nagbar.
(Presumably ClamAV?)