Gmail Labs has been very active in the past few days. Recently they launched the auto reply feature in Gmail and now they are back with Calendar and Google docs integration in Gmail. This is an experimental feature and available as gadgets to the left-navigational area, next to Chat and Labels. For using this features, you need to enable this in the labs settings.
There is one more feature added to Gmail Labs which is pretty exciting one, add any gadget to Gmail using the URL of its XML spec file. With this feature you wont be need the Firefox add-ons to get more services working in Gmail, it can be directly integrated to Gmail through this feature.
Gmail is getting more and more exciting feature with labs. The add gadget by URL is a feature definitely worth trying.