Darkness is a world clock on steroids. Not only does it give you the current time for pretty much any city in the world, it also lets you know what time the sun rises and sets, alongside other specific information such as the the phase of the moon.
Darkness 2.0 is a complete rewrite, sporting a new interface and some enhanced features. For photographers, Darkness is a really, really useful tool because it tells you where the sun or moon will be at any given time. you can also find out the exact time that solar noon (the time of day when the sun appears its highest) will occur in your location, so you can help plan for the best time to take certain shots.
Darkness offers up a nice way to quickly glance at what time it is in various places all over the world. Sure, the built in World Clock can do the same thing, but Darkness gives you more accessible information (the day for instance) and can access your current location, which is great if you happen to be traveling across timezones.
You can also easily choose to view the time in military or 12-hour intervals, and tell at a glance at what time sunrise and sunset is in any given city. You can add your own city from Bjango's large database (more than 8500 cities) or you can enter in your exact coordinates if you live off the grid or something.