Hardware keylogger
A small physical device that sits between the keyboard and the computer and logs every keystroke — invisible to the OS and antivirus.
The USB pass-through version looks like an extension dongle. PS/2 versions are even smaller. Some come with Wi-Fi to exfiltrate data wirelessly. Sold openly for "parental monitoring" or "lost-password recovery" but also used for espionage. Defences: visually inspect the connector (anything unusual between keyboard and port), wireless keyboards (harder to hijack physically but vulnerable to KeySniffer attacks), full-disk encryption (limits what someone can do with stolen passwords).