If you are anything like me, you probably sign in to Gmail from multiple computers. I, for example, occasionally sign into my Gmail account from a friend's house when I need to check an important email. Usually I remember to sign out, but every once in a while I wonder if I really did. Now I no longer have to wonder.
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:
Using the above example, a quick glance at the footer while I'm at work shows that my account is indeed open in one other location. But I recognize the IP address - it's my computer at home. Turns out I left my Gmail account open when I left home in a hurry this morning.
Your IP address, or Internet Protocol address, is a number associated with your computer when you connect to the Internet. Some of you might want to know your IP address numbers (more info on finding out your IP address below) for an extra layer of control. For others who don't want to think about IP information, you don't need to.
Continuing with the above example, I can see more details about my recent activity by clicking on the Details link:
The top table, under "Concurrent session information," indicates all open sessions, along with IP address and "access type" -- which refers to how email was retrieved, for example, through iGoogle, POP3 or a mobile phone. The bottom table, under "Recent activity," contains my most recent history along with times of access. I can also view my current IP address at the very bottom of this window, where it says "This computer is using IP address..."
With this information, I can quickly verify that all the Gmail activity was indeed mine. I remember using Gmail at the times and locations listed. Being extra cautious, I can also click on the "Sign out all other sessions" button to sign out of the account I left open at home.
Note: We are in the process of rolling this feature out to the latest version of Gmail, which is available for Firefox and Internet Explorer 7.