Georgetown Steam Power Plant Museum
The Georgetown power plant is a well kept secret of Seattle. Its only open once per month and is really not advertised. I constructed a Photosynth of this space:
http://photosynth.net/view.aspx?cid=5200d087-73d7-4fe2-ad5b-89cf459d69a5
I think I see what Photosynth is trying to do (a 3D walkthrough slideshow), but wow is that UI confusing!