The first round of transmog gear for purchase has appeared in the Blizzard store. The Crown of Eternal Winter, Jewel of the Firelord, and Hood of Hungering Darkness are all head slot items with zero stats, functional only for cosmetic reasons. All three of these items are unique models with animations, and I do have to say, they are rather visually arresting. Of the three, I think the Jewel of the Firelord (pictured above) is my favorite, but I'm quite sure the Crown of Eternal Winter will be a popular choice thanks to its skull face look. Spooky!
Something notable about these helms is that they don't seem to have an armor type. The description tells us that they work across all races and classes, and are available on all current and future WoW characters. So for those of us playing classes that might not have a whole lot of variety available to us (ahem, mail-wearers) this could be a nice way to expand the wardrobe.
All three of these helms are available for purchase in the US Blizzard Store under the Collectibles tab for $15.00 apiece and in the EU Blizzard Store under the Pets & Mounts tab, for 12,00 euros each.