The first in what is planned to be an ongoing stream of content for Elder Scrolls: Oblivion has been released.

Entitled The Orrery, the content is a new Mage's quest for Cyrodiil. You must help the Mages repair the Imperial Orrery. Parts have been stolen and you must retrive them and return them to the Arcane University.

From Elderscrolls.com
The Orrery allows you to help the Mages Guild of Cyrodiil repair the Imperial Orrery, an ancient dwarven machine with mystical properties. Bandits have stolen a shipment of parts destined for the Arcane University; if you can return them, the Orrery will function once more.

Features

* A new, full-featured quest, including dialogue, journals, and rewards
* Access to the Imperial Orrery in the Arcane University, an incredible mechanical marvel
* New powers available for your character based on the phases of the moons

The Orrery will cost 150 Microsoft points, and is 9.83MB in size.

Also planned for a future release, and described in more detail on the Elder Scroll's forum is The Wizards Tower, a new building full of items for the more magical adventurer. No release information is forthcoming for this download.

Related Links
Britxbox's Oblivion Review
ElderScrolls.com Forum