Sometimes the most simple and obvious feature can be the most secure as Google is rolling out a feature that allows you to monitor remote activity on your Gmail account.
With this new service, Google will monitor the Internet protocol addresses of every computer used to sign into your Gmail account. It will then display that information at the bottom of your main page including how many locations your Gmail account is logged in to, as well as the last activity time.
Check out a list of PC World Gmail Tips.
Remote Log Out Feature
This is a simple update that could greatly increase security. Gmail's Web interface makes it easy to log in at many different computers, including those at public places like coffee shops and libraries. The best security bet is to always remember to log out of your Gmail account at all computers, but if you forget, this service will also allow you to force a log out at that computer using an "Activity on this account" prompt.
The one negative taken from this service is that now the ultra-paranoid have one more item to constantly monitor and check. Oh well, it's better to be too safe than not safe enough, especially when dealing with e-mail.
Google is rolling out this feature across all accounts, so it may not appear in your account yet.