Earlier this month, we told you about a Google field trial which incorporates results from Gmail and Google Drive in your search. If you've signed up for the trial, you can use keywords to restrict your search just to Gmail or Drive.
Google Operating System points out that if you start your query with "gmail'", you'll get just results from your account, and if you use "drive", you'll see Google Drive results first. As with the trial itself, availability of the feature varies: on some occasions it has worked well for me, on other occasions Google ignores the keyword. When it does function, it's a useful option.
Show Gmail and Google Drive Results in Google Search [Google Operating System]