How do you output the \ symbol using cout?

In addition to all the correct answers, see this for further escaped characters

\a  Bell (beep)
\b  Backspace
\f  Formfeed
\n  Newline
\r  Return
\t  Tab
\\  Backslash
\'  Single quote
\"  Double quote
\xdd    Hexadecimal representation
\ddd    Octal representation
\?  Question mark ('?')

Use two backslashes \\