Now you can experience the power of Google Search in Gmail; just turn this feature on from the Labs tab under Settings, you'll see a new search box on the left side of your inbox: — Type your search, and a window appears at the bottom with the first few search results. You can click on a result and it'll open up in another window (or another tab) so you can make sure it's what you're looking for. Once you're sure it's a result you need, moving your mouse over the result back in Gmail reveals a pull-down menu that lets you do stuff with the search result.
If you have keyboard shortcuts turned on, typing g and then / will take you to the search box when you're not composing, and Ctrl + g will do it when you're composing (that's ? + g for Mac users), explain Gmail blog.