What a great idea. I can think of many times I wish I'd had this utility.
SniffPass is small freeware utility that listens to your network, capture the passwords that pass through your network adapter, and display them on the screen instantly. SniffPass can capture the passwords of the following Protocols: POP3, IMAP4, SMTP, FTP, and HTTP (basic authentication passwords).
You can use this utility to recover lost Web/FTP/Email passwords.
http://www.nirsoft.net/utils/password_sniffer.html
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(http://www.nirsoft.net/utils/password_sniffer.html)
from http://www.all4you.dk/FreewareWorld