Google have added a cool feature for users of GMail – the ability to sign out from previously logged in sessions – therefore if you have the habbit of signing into GMail from multiple PC’s and "forget" to logoff, scroll to the bottom of you current GMail screen and you will see the new feature titling "Last account activity: xx minutes ago on this computer. Details.."
Click the <details> link and you’re presented with a list of previous sessions which allows you to quickly verify that all the GMail activity was indeed yours. To be extra cautious you can click on "Sign out all other sessions" – this way you prevent any unauthorised usage of previous sessions.
Full report can be read here