I've enjoyed the temporary wall art some construction sites are now putting up to amuse passersby...
but a public art program at the bus and train stops is rockin' the temporary art hard, I'm tellin' you. STart (Sound Transit Art)'s Red Wall project curates an "art lab" that surrounds the light-rail construction in the Capitol Hill neighborhood. Some of my favorites as I walked around the site:

That one above is part of an experimental animation project. (With a couple of nods to "Northern Exposure.")
The pointilist portrait above was created with tons of these:
You do have to back up to see the portrait, but I wouldn't advise backing up all the way out into the street.

STart also has a Flickr album for the Red Wall project, as well as its other public art projects. Any cool temporary art projects in your neck of the woods? Tell me tell me tell me.