Brewfest has started. Until October 5th you can get royally drunk, race rams, fight iron dwarves, and pray for the random number generator gods to smile at you. Wowhead has up up a comprehensive guide to everything Brewfest. Get your achievements, your new fancy 365 item level loot, and maybe even a mount or two!

» SourceBrewfest Guide: Achievements
Brewfest is more than just standing around dressing funny and drinking grog! In the midst of the unruly behavior and excessive consumption of alcoholic beverages, there's a title to be earned--Brewmaster. For many players, completing the Brewfest achievements will finally earn them What A Long, Strange Trip It's Been, so expect to see even more Violet Proto-Drakes flying around.

Brewmaster
Complete all of the Brewfest achievements except Brew of the Year, Drunken Stupor, Strange Brew and Disturbing the Peace and you will be awarded the player title 'Brewmaster'. That's right, with the addition of Brew of the Month, players are no longer required to spend an entire year waiting to complete the Brew of the Year achievement for What A Long, Strange Trip It's Been.

Brew of the Month
For this achievement, you must purchase a "Brew of the Month" Club Membership Form for 200 Brewfest Prize Token from either Blix Fixwidget, Ray'ma, Belbi Quikswitch or Larkin Thunderbrew at the main Brewfest camps or inside Orgrimmar or Ironforge. This will entitle you to receive a special brew every month, for members only. You will be able to purchase more of a specific month's brew from a Brew Vendor located inside Orgrimmar or Ironforge.
Brew of the Year
For this achievement, you must drink one of each of the 12 beers featured in the Brew of the Month club. You will receive a new brew at the beginning of every month and will only be able to purchase that brew for the duration of that month--be careful you don't miss one. This achievement will take a full year to get, but thankfully it is no longer required for the Brewmaster achievement.
Direbrewfest
For this achievement, you must travel to the Bar in Blackrock Depths and defeat the Dark Iron dwarf Coren Direbrew.
Disturbing the Peace
For this achievement, you must purchase and wear three pieces of Brewfest clothing (any holiday hat, boots, and chest), get completely smashed from the alcohol of your choice and /dance in Dalaran.
Does Your Wolpertinger Linger?
For this acheivement, you must complete the quest Catch the Wild Wolpertinger! or Catch the Wild Wolpertinger! and obtain the Wolpertinger's Tankard. Enjoy your cute and creepy pet!
Down With The Dark Iron
Defend the Brewfest camp from a Dark Iron attack and complete the quest, This One Time, When I Was Drunk... The Dark Iron attacks happen roughly every half hour at either of the main Brewfest camps, located outside of Orgrimmar or Ironforge. Once you have 'defeated' the Dark Iron attack, simply loot the Dark Iron Mole Machine Wreckage which will start the required quest, which can be turned in nearby.
Drunken Stupor
This achievement is just a Brewfest version of Going Down?, so any strategies/locations specified for that will apply. Just remember to get really, really intoxicated beforehand (ie. "completely smashed") and try to survive the fall.
Have Keg, Will Travel
For this achievement, you can now either own a Brewfest mount (either dropped by Coren Direbrew or obtained via tokens in 2007) or simply change one of your existing mounts into one by using Fresh Brewfest Hops, which can be purchased from Brewfest vendors for a mere 2 Brewfest Prize Token. You can also use Preserved Brewfest Hops, though these will cost more Brewfest Prize Token.
Strange Brew
For this achievement, simply sample all of the beers available from your faction's Brewfest vendors. These beers can be purchased at the Brewfest camps found outside of any major city. Horde players will purchase brews from T'chali's Voodoo Brewery Apprentice, Drohn's Distillery Apprentice and Gordok Brew Apprentice. Alliance players will purchase brews from Thunderbrew Apprentice, Barleybrew Apprentice and Gordok Brew Apprentice.
The Brewfest Diet
For this achievement, simply sample all of the foods available from the Brewfest vendors. Horde players will purchase food from Bron, Agnes Farwithers and Uta Roughdough. Alliance players will purchase food from Keiran Donoghue, Anne Summers and Arlen Lochlan.
A Brew-FAST Mount
This achievement is a Feat of Strength, so it will not award Achievement Points or count towards any other achievement progress. This achievement is awarded for obtaining either of the epic Brewfest mounts tied to Coren Direbrew in Blackrock Depths - either the Great Brewfest Kodo or Swift Brewfest Ram. Good luck!

Links
Wowhead - Brewfest Guide