Flour Art Museum in Ahrensburg, Germany
I know where I'm going the next time I'm in Germany: the Flour Art Museum in Ahrensburg. The museum grew from the collection of Volkmar Wywiol, owner of flour production company Mühlenchemie, who started his collection when he found an empty flour sack washed up on the beach in Dubai in 1998. With the help of Mühlenchemie's customers and employees, the collection now includes "over 1,900 sacks from more than 115 countries" (and if you think you have something they don't, they take contributions). The sacks are arrange alphabetically by country of origin in the appropriate named Sackotheque section of the museum.
The museum's website has more information about the history of flour and the language of flour sacks. You can also order their accompanying book, Art and Flour - a Worldwide Gallery of Flour Sacks. [via Wish You Were here and NOTCOT.ORG]