If you left yourself logged in at an internet cafe or on a friend's computer, don't worry: you can see and remotely log out all of the computers and devices on which you are currently logged in.
Note: This screen only shows logins to our web interface. Email clients connecting via IMAP/POP/SMTP do not appear because they do not create a long-lived session. They need your password every time they connect. If you left yourself logged into an IMAP client somewhere, you can revoke access by changing your password. This will immediately kill all connections to your account. When they try to reconnect, the clients must use the new password.
At the bottom of the Settings → Password & Security screen is a link to view the history of every time you logged in to your account over the last two weeks. This includes all protocols (web, IMAP, SMTP etc.) and will also show any failed attempts to log in to your account.