The Elder Scrolls Website has word that the Dragonborn DLC for The Elder Scrolls V: Skyrim is now available on Steam for the PC edition of Bethesda's RPG sequel. This page has all the details on the add-on, and here's the overview:

With this official add-on for The Elder Scrolls V: Skyrim, journey off the coast of Morrowind, to the island of Solstheim. Encounter new towns, dungeons, and quests, as you traverse the ash wastes and glacial valleys of this new land. Become more powerful with shouts that bend the will of your enemies and even tame dragons. Your fate, and the fate of Solstheim, hangs in the balance as you face off against your deadliest adversary – the first Dragonborn.