BART, in partnership with San Francisco-based developer Junaio, released its first official augmented reality app--or, more accurately, its first official augmented reality layer.
Augmented reality adds text or graphic overlays onto real objects as seen by a phone's camera, and while it's often of questionable use (like, say, turning your neighborhood blue as an homage to Avatar), it has a lot of potential to help people find specific locations faster and easier than looking at a map. Typically, individual searches will have their own layers--there might be a pizza restaurant layer, or a hotels layer--and Junaio has added a BART layer to its own iPhone app, so you can always find a nearby BART station, along with estimated arrival times (which are impressively accurate).
Junaio adds a few features not seen in the leaders in the augmented reality category, like Layar: its best might be the element of interactivity. You can add little notes for friends or whoever wants to see them--take a picture of your favorite coffeeshop, write a quick note ("great service, macchiatos are incredible!"), and post it, so the next time somebody walks out of a nearby BART station and wants coffee, they'll see your note pointing them in the right direction.
Junaio is free, but only available for the iPhone 3GS (not 3G or 2G) at the moment. A version for Android is underway.