Google tasked its engineers from the Calendar and Docs teams to come up with two highly requested features. They include a simple way to see your Google Calendar agenda and get an alert when you have a meeting (something Microsoft's Outlook does), and a gadget that shows a list of your recently accessed Google Docs and lets you search across all of your documents right from within Gmail.
The addition of the Calendar notifications is excellent. It isn't the biggest deal to have to open the Google Calendar to see when your next meeting is, but automatic notifications are a welcome addition. Since I routinely use Google Docs to share documents with my colleagues, having the ability to peruse my files from within Gmail also is a nice time saver. It cuts out the step of having to open up my Docs home page.
Aside from these two, Google added a third Lab. This last Lab feature allows users to add any gadget by pasting in the URL of its XML spec file. Google writes, "We realize this isn't very user friendly right now; it's a sandbox mainly aimed at developers who want to play around with gadgets in Gmail. We're not tied to the left-nav as a primary way to extend Gmail -- in fact, we think it is relatively limited and doesn't offer scalable real estate. There are also some downsides to the iframe-style Gadgets we're using today -- they can sometimes slow down the page. We're fanatical about speed, so we'll be keeping a close eye on performance."
In the meantime, I highly recommend that you check out the new Calendar and Docs gadgets in Labs. They are well worth enabling.