Filled Green Brewfest Stein

The can be filled at a Brewfest Festive Keg, becoming a Filled Green Brewfest Stein, and then can be used to drink the brew. Filling, hence drinking, is free, and the brews are both alcoholic (inebriating) and provide mana restoration and a well fed buff.

Source
During Brewfest 2009, the Green Brewfest Stein was a quest reward from:

Use
Equipping an empty Green Brewfest Stein, then using a Brewfest Festive Keg will convert the stein into a Filled Green Brewfest Stein, containing a one charge sample of the beer from the barrel. The filled stein can then be used, drinking from it, which converts the stein back to an empty Green Brewfest Stein. The imbiber get the effects of the particular beer that was sampled. The full or empty stein must be equipped to be used.

Varieties
Each of the three Brewfest Festive Kegs contains a different sample brew, so, depending on what barrel the owner fills their stein at, their Brewfest Stein will have a different sample of brew. Each full stein with a different brew is a different item, as is the empty mug. There are two icons, one for the empty stein and one for the full steins.

Each Brewfest Festive Keg is near to one of the brew vendor tents. Each brewer vends three different strengths of brew, but the breweries specialize in a low, medium, or high strength range of brews. Stronger brews are higher level, restore more mana, have stronger buffs, and have a higher character level requirement to use. Each of three brews available from the Festive Kegs are the same as one of the three brews being vended by the brewer nearest that keg.

The Alliance and Horde brews are different brews but have equivalent effects. The Barleybrew dwarves' and Drohn's orcs' brews are the weak, low level range. The sample nearest them is the level one sample, the weakest at the Brewfest, and can be sampled by all attendees. Thunderbrew dwarves and T'chali's trolls have mid-range brews. The Gordok Ogres are shared by both factions, and have the strongest, highest level brews.

Note: The Festive Kegs are neutral and inanimate. You should be able to use the opposing faction's Festive Kegs, if you can get to them.

You can fill your mug with any of the three brews. The only way to empty your mug is to drink it (use it). Drinking from your stein and refilling at a different Brewfest Festive Keg switches between brews.

Anyone can fill their stein from any of the kegs. Since each beer has a minimum level associated with it, but the stein does not, you can drink a brew from your mug that has a higher level requirement than you are, but you will not get the buff. Instead you get a brief "Can't Hold Your Brew" debuff with a 'getting sick' graphic and a message warning you not to drink the big kids' drink.

Note that the Alliance/Horde brews may be available in neutral cities as well.

Quotes
This is the flavor text for each brew. Note that the quotes for the 2009 Stein are the same as the quotes for the 2008 Stein.

Alliance

 * Barleybrew Clear ( "Barleybrew brew, when you want the very best." )
 * Thunder 45 ( "You've called down the thunder!" )

Horde

 * Small Step Brew ( "Drohn's Distillery, a wise choice." )
 * Jungle River Water ( "T'chali's Voodoo Brew has magic you don't want to understand." )

Both

 * Gordok Grog ( "Gordok brew, it's wet." )

Dark Iron Dwarf invasion
You fill at the same Brewfest Festive Kegs that the Dark Iron Dwarves attack when they invade. If the Dark Iron Dwarves succeed in destroying a Brewfest Festive Keg, you will be unable to fill your mug from that keg until it respawns. The destroyed keg will get a destroyed graphic with a caved in front, then disappear as the Dark Iron Dwarves depart.

Uniqueness
The Green Brewfest Stein is unique. Since the empty and filled forms are different, uniqueness might not apply, but the mug is soulbound and you only get the quest once in 2009, so you'll only have one anyway.

Use as a weapon

 * This mug is a miscellaneous weapon, as was.
 * It must be wielded to be filled and used, so presumably is miscellaneous so that any character can use it as a mug.
 * As this item is classed as a weapon, attempting to use when it is not equipped it will equip it.
 * Both the full and empty forms have the same low weapon stats.
 * Since they are miscellaneous type weapons, many class skills will not work with them.
 * On the other hand, they have no level requirement, and neither does the quest that awards the mug.
 * Literally any character can obtain and wield these in the current year.
 * If you have the mugs from either 2007 or 2008 and acquire the 2009 mug, you can dual wield them (assuming you can dual wield).

See Foodfight

Disenchantment
According to Blizzard's The Armory, (see external links, below), any of the six Blue Brewfest Stein items can be disenchanted, yielding a. However tests say that it can not be disenchanted at all.