Google have made Fuse work on OS X (project page). This means that it is now trivial-ish to implement all sorts of file systems on OS X. The big deal is that you can do it as a regular program, rather than as a kernel extension.
One of the most useful ones are sshfs that lets you mount a directory on another server over ssh. Any box you can ssh to, you can now mount on your mac! The Transmit / TextMate integration is nice, but this is about a thousand times better. Someone already made a sshfs OS X binary.
Yay, this is Really Cool.