No kqueue is not an acceptable alternative as it uses file descriptors to "watch" for changes :(

There's also the syncthing port and owncloud sync support via mirall.

That's a backup solution which you can also run on Linux without an issue. Dropbox is more for collaboration. It's often the only tool in its class used by smaller publishers or marketing companies. They don't have an ftp or webdav server. And it sure beats emailing files around.

I use Tarsnap and Dropbox. They aren't the same at all:

* You run Tarsnap whenever you want to push changes; Dropbox automatically pushes changes.

* Tarsnap is for 'real' computers only; Dropbox runs on my phone (which is very useful for photos, among other things).

* Tarsnap is billed by usage (at a very small amount); Dropbox has a free tier.

* Tarsnap doesn't have a natural sharing usecase; Dropbox does.

* Fundamentally, Tarsnap is backup, and Dropbox is sync. They are two different things. Not to mention that Tarsnap is also supported just fine on Linux.