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.
I don't see Siracusa's manifesto as a specification for an application - though it could be turned into one. Rather I see it as a collection of related concepts which could lead to a more intuitive use of our Macs. Some technologies and applications which have been implemented in the 10 years since the article was written include:
Metadata. Tags (in the form of OpenMeta tags) have been around for some years and have now been incorporated by Apple into OS X. The most capable application using tags is arguably Leap.
Saved Searches: These have been part of Finder for a while along with the ability to search on metadata. When combined with tags (and other metadata) these provide new and powerful views of files.
Quick Look and Cover Flow: Again part of the existing Finder these provide a visual view of files which is part of Siracusa's concepts. I must admit I seldom use Cover Flow, but Quick Look is an essential part of my use of Finder.
DesktopShelves: The shelf concept has been turned into an application. One of its uses is a temporary location (a shelf on the desktop) when moving files around. I also like to keep Saved Searches on a shelf (rather than the Finder Sidebar).
So rather than a single application implementing all of Siracusa's ideas, we can see that there are tools and technologies which have incorporated some of his key ideas.