So often, writing an email involves opening a new tab to look up something, copying the link, then heading back to paste it. Gmail's latest Labs feature consolidates that process with pop-up Google searches.
Once you enable the Google Search feature in Gmail's Labs menu, you'll see a new Google Search bar in Gmail's left-hand sidebar. Depending on whether you're in the inbox, writing a new mail, or replying to someone else's message, what happens after you type your search and hit enter varies a little bit, but it's all meant to help you search and insert direct links and search result references in your emails.
So, for example, when you're composing a new email, searching from the left-side box will pop up a window just like a chat message with the first few results. Hover over the bottom-right of any result, and a drop-down menu lets you paste that result's direct URL (instead of the common Google click-tracking gobbledy-gook), paste a link to the Google search results page, or pop open a new message with that link auto-inserted. Search from the inbox, and you can start a new message with a search result.
Pretty neat stuff, and pretty handy in certain cases, too. Gmail's making it easier and easier to do more and more from a single browser window.