Usefully Reflective Windows

Look at the nearest window-covered building: not all the windows face precisely the same direction, so the reflection of the clouds is all messed up. Sometimes you'll get a blinding reflection of the sun from several slightly misaligned windows.

Put the windows at precisely the right angles (not 90 degree angles) and give each one a slight curve (parabola, etc.) so that all the sunlight focuses on a single point. Sunlight comes from the right direction only once a year, so the windows might have to move to adjust for time and season. This might look cool even without the sun.

The focus could be centered in a tub of water that would heat up and turn a turbine generator, etc. Or it could focus on a key element those industrial lawn sculptures that many businesses put on the grass. You could focus on a big disco ball, although it might not be as special during the daytime.

Or my favorite: building wars. Law firm across the street taking your business? Cook the competition! I'm not serious here, but perhaps there are military applications. Maybe tall buildings could shoot down incoming terrorist planes (if you don't mind killing the civilians) with focused sunlight. The larger the building, the more likely it is to be a terrorist target, the more power it would have to protect itself! Imagine a bank robbery: melt the getaway car (you might have to spray it with black paint first, curse those white, unmarked vans)!