Sour beers have been gaining popularity in the West Coast. I've read about many breweries adding space to age more sour beers. Here in New York State it can be tough getting hold of such beers. Most of the best examples are made in Belgium or in California; the Cantillon and Russian River breweries, respectively, are marked examples.
There are other breweries that are closer, but finding their beers can be hit or miss depending on the State. Those breweries that I am aware of include Jolly Pumpkin in Michigan, New Belgium in Colorado, and Allagash in Maine. Our Ommegang brewery in Cooperstown, NY is an excellent Belgian-style brewery, but they have not gotten hip to the sour beer thing as far as I know.
I'm not going into the specifics of sour beers here, but suffice it to say they tend to have some tart and funky flavors, they are fermented with wild yeast and bacteria, and require longer aging periods.
Appearance: Golden, almost orange color. Moderate carbonation.
Aroma: Green apple. Rotten apples. Peach. Baby food carrot puree.
Taste: Tart apple and subtle funk. Some sweet beer malt, as the beer warms. It's easy to taste a range of subtle fruit flavors.
Mouthfeel: The beer has a nice thick mouth-feel. There is a dry, lip smacking finish.
Overall Impression: I liked it. There is the Brett sourness and funk, but not as low a pH and overbearing funk as the Lindemans Gueuze Cuvee Rene I tasted. This beer has a thicker mouth-feel and some malt sweetness. The level of sour/tart is approachable. The flavor improves as it warms. Seems like a good starter for newbies.