Is there software that implements John Siracusa's Spatial Finder concept?
This isn't a direct implementation of Siracusa's vision. It is, however, a Finder alternative that looks distinctly spatial in orientation, an the opposite of the file browser direction most other Finder alternatives (and the Finder itself) seem to be going:

Raskin for Mac seems to be the very spirit of the spatial Finder that Siracusa has been calling for. I just downloaded the free trial and am exploring it, but this might be exactly what I've been looking for.