Many tricks can let you change, delete or recover lost passwords.
Depending on your operating system, you can either reset or recover your password. Before proceeding, you need to know your operating system
Some softwares remember your own passwords on the computer. This is the case of Outlook Express or Thunderbird when you have already used these softwares in the past to manage your emails, or Windows Live Messenger when it automatically connect.
Recover the passwords stored by Thunderbird (soon)
Recover the passwords stored by Outlook Express avec MailPV (soon)
Recover the passwords stored by Internet Explorer (soon)
Recover the passwords stored by Firefox (soon)
Recover the passwords stored by Google Chrome (soon)
Recover the passwords stored by Mac OS X (soon)
Wireless networks passwords
Your PC also store the password of a wireless network on which you connected. And if the wireless network is not well secured, it is possible to crack its password, hence the importance of properly securing your wireless network.
Recover your wireless network password stored on Mac (soon)
Crack your WEP wireless network password on Ubuntu (soon)
Online services passwords
Except installed softwares on your computer that could still remember your password for an online service such as Skype, there is no magic way to recover the password for a online service except those that the service offers itself. You should go on the website of that service, search for forgot password near where you can login and follow the instructions. There is nothing else you can do.