I absolutely love the Keep Remote Directory Up-to-date feature in Winscp. Unfortunately, I can\'t find anything as simple to use in OS X or Linux. I know the same thing can
You can also use Fetch as an SFTP client, and then edit files directly on the server from within that. There are also SSHFS (mount an ssh folder as a Volume) options. This is in line with what stimms said - are you sure you want stuff kept in sync, or just want to edit files on the server?
OS X has it's own file notifications system - this is what Spotlight is based upon. I haven't heard of any program that uses this to then keep things in sync, but it's certainly conceivable.
I personally use RCS for this type of thing:- whilst it's got a manual aspect, it's unlikely I want to push something to even the test server from my dev machine without testing it first. And if I am working on a development server, then I use one of the options given above.