Gmail is rolling out a new feature that allows you to view information about the last time your account was accessed and whether it is currently open on another computer.
At the bottom of your inbox, you'll see information about the time of the last activity on your account and whether it's still open in another location:
You can view more information about your recent activity by clicking on the details link
You can see open sessions, when they were accessed and how the were accessed, by browser, POP, phone.
It is not yet activated on all accounts. I will be really excited when it is activated on Google Apps.