Thought I’d post this here for future reference: http://www.cis.upenn.edu/~bcpierce/unison/.
I have had occasion to go looking for it more than once in the past. It’s quite a nifty tool that actually works on (and across) a list of many platforms, which happens to also include windows. As stated on the site it’s not just a mirroring app, but a bi-directional file-synchronization program that uses the “rsync algorithm”.