Gmail makes it easy to find emails with Google-powered search for inbox. But what if you want to search information in documents and sites; Google now made it easier to search across documents using "Apps Search" lab for Gmail. "Once the lab is enabled the "Search Mail" button in Gmail will say "Search Mail and Docs" instead, and your search results will include matching documents and sites in addition to email messages.
Also added "Did you mean?" style suggestions.
Since, this is a Labs launch, to get started, click "Settings" within Gmail, then "Labs" tab, and enable "Apps Search".
This feature is also released to Google Apps – Standard Edition users can follow above instructions immediately, while Premier and Education Edition users will first need their domain administrators to enable Gmail Labs from the Google Apps control panel," explains Google.
More Info: Help Center