Integrated Gmail is a Firefox extension that adds Google Calendar and Google Reader to Gmail's interface. It's a simple way to switch between the three Google services without opening multiple tabs.
The extension is customizable, so you can add other Google services, reorder them and delete the ones you don't use. The interface for each service loads in a collapsible box and it's preloaded when you open Gmail.
You can add events or check your agenda while composing messages, read your feeds and chat with your friends from the same interface or copy some text from an email to a notebook.
The extension is not perfect, it might slow down browsing, but it's an interesting experiment and a good starting point for an official unified interface for Google's services. iGoogle, Google's iPhone interface and the support for gadgets in Gmail show that this idea has a great potential.