Last night was my first exposure to a sour stout. Jolly Pumpkin's Madrugada Obscura. Bizarre stuff and it was delicious.
To continue on Reaper's line of information about sours, Jolly Pumpkin uses wild yeasts in all of their beers, so they end up with sour versions of a lot of typically non-sour beers. Hence, the sour stout I referenced. They also had a witbier, a brown ale, a pumpkin beer, a christmas ale, and a saison. All were tart at the least, some were full on sours.