Google today launched a new Gmail Labs feature called Apps Search that when enabled lets you search both your inbox and Google Docs straight from Gmail.
Once enabled, your Search Mail button is replaced by a Search Mail and Docs button. When you perform a search, you'll see a selection of matching documents from Google Docs below your mail results. To enable it, head to the Labs tab in Gmail, find and enable Apps Search, and click the Save Changes button at the bottom of the page. Gmail will reload, and the handy new Apps Search will be up and running in your inbox.
Search across your docs, sites and email with the Gmail "Apps Search" lab [Google Enterprise Blog]