Gmail Team blog announced the addition of a new security feature to Gmail “tracking open sessions”, with which you can now track your recent sessions and you can also sign yourself out remotely. The feature is currently being rolled out in the new version of Gmail, so you may not see it on an older version.
If you sign in to Gmail from more than one computer and you forget to sign out, now 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:
With this feature, now you can sign out remotely from all the locations where your Gmail account is still open.
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...”
This could be useful if you want to find whether someone else has access to your account: you'll be able to find the IP address and the date of the most recent activity in your account.