The FUSE (Filesystem in User SpacE) system has been ported to the Mac by a Google employee or three.

Why do you care?

Well, FUSE lets you mount new filesystems at the user level (not the root level). These are actually virtual filesystems, as the FUSE code permits you to rather abstract what it means to be a filesystem, and thus use, say, your Gmail account as a filesystem. (So you can have multiple Gmail accounts, each granting you 3 gigs of online storage for free.)

Other cool tricks include simply supporting unsupported filesystems (crytoFS), improving support of already supported filesystems ( add read/write to NTFS, improving WebDAV performance) or even supporting different sorts of remote servers (such as FTP or SSH) as filesystems (permitting you to securely access your home computer as a “disk” using nothing more than the included SSH server).