Curious how many emails you sent last month? What your most frequent search term was? What countries you logged in from? If you use Google for everything, it can now tell you all of those things.
Google's new Account Activity feature not only serves to answer your curiosities about how you use Google's services, it could also tip you off to hackers/phishers/scammers who might be in your account (see the report section on logins and authentications).
To enable the new Account Activity report, just click this link, make sure you're logged in, and choose to opt in to the new feature. It will begin generating a report right away, and send you an email when one is ready. [Google]