I just noticed this announcement on the google mac blog.
Amit Singh has released a mac version of Fuse, which is way for people to write interesting extensions to the file system without writing kernel code. The way it works (as I understand it) is that the Fuse system runs one kernel module which communicates with the actual file system code in userspace.
It looks really clever and there are already a number of useful file systems available for fuse (including ssh as a filesystem and a ntfs driver).
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