A growler is a type of beer bottle that allows someone to take home draft beer from a brewery (or any other place that happens to have beer on draft for that matter; e.g. bars). They have become very popular in places like Atlanta, where you can fill up at many liquor stores, but not very popular in California because, under state law, breweries can only fill growlers with their name on it. For example, under the existing law, you cannot go into Linden Street and get them to fill your Drake's growler with Linden Street beer. Liquor stores filling growlers is obviously out of the question since they're usually not brewing anything.
In 2011 and 2012, there was a push to reform this law, and on October 10 2013, Governor Brown finally signed Assembly Bill 647 (AB-647 The Alcoholic Beverage Control Act: beer manufacturers: containers). The law takes effect on January 1, 2014, but you may find that many places are operating as if the law is already in effect.
Growler Fill-Up Costs
Alameda Island Brewing Company: $10 or $25
Faction Brewing: $15 to $19
Linden Street Brewery: $10 to $15
Pacific Coast Brewing Company:
Sierra Nevada Torpedo Room: $13 to $22 (won't fill non-labeled growlers)
Temescal Brewing: $19 or $23
See also AB933.
The California Growler Initiative (Mutineer)
Where to Fill Your Growler in Metro Atlanta (The Trot Line)
California Beer Bill Would Permit Growler Refills at Breweries (East Bay Express)