Mail PassView is a small password-recovery tool that reveals the passwords and other account details for the following email clients:

Outlook Express
Microsoft Outlook 2000 (POP3 and SMTP Accounts only)
Microsoft Outlook 2002/2003 (POP3, IMAP, HTTP and SMTP Accounts)
IncrediMail
Eudora
Netscape 6.x/7.x
Mozilla Thunderbird
Group Mail Free
Yahoo! Mail - If the password is saved in Yahoo! Messenger application.
Hotmail/MSN mail - If the password is saved in MSN Messenger application.
Gmail - If the password is saved by Gmail Notifier application.