After having to pull the gadget because of some security issues, the Live.com blog has just announced that it's back. The gadget adds a view of your Windows Live Hotmail account to a Live.com page. You must be using Windows Live Hotmail to use the gadget, it won't work with an old-style Hotmail account.
Add the Mail gadget to your Live.com home page
More on why the gadget was pulled in the first place.