This is the black and white flag of Brittany, a province of France. I don't think I've ever seen a black and white flag before, but the Wikipedia entry notes that there's a few locales that have black and white flags. Via.
While looking for photos of the flag, I found this image at Flickr:
It has something to do with a pilgrimage called the "Tro Breizh." Wikipedia explains:
In Brittany, there is a very old pilgrimage called the Tro Breizh (tour of Brittany), where the pilgrims walk around Brittany from the grave of one founder saint to another. The seven founder saints of Brittany are:
* St Pol Aurelian, at Saint-Pol-de-Leon/Kastell-Paol,
* St Tudual (sant Tudwal), at Tréguier/Landreger,
* St Brieuc, at Saint-Brieuc/S-Brieg,
* St Malo, at Saint-Malo/S-Maloù,
* St Samson of Dol, at Dol,
* St Patern, at Vannes/Gwened
* St Corentin (sant Kaourintin), at Quimper/Kemper
Historically, the pilgrimage was made in one trip (a total distance of around 600 km). Nowadays, however, pilgrims complete the circuit over the course of several years. In 2002, the Tro Breizh included a special pilgrimage to Wales, symbolically making the reverse journey of the Welshmen Sant Paol, Sant Brieg, and Sant Samzun. Whoever does not make the pilgrimage at least once in his lifetime will be condemned to make it after his death, advancing only by the length of his coffin each seven years.
I wonder why there's a slash though the sign.