Unfortunately, there don't appear to be any good open-source solutions at the moment. I'm not looking for a fancy web interface or anything, just a simple sync between devices, with a usable API for building apps.
In particular, rsync etc. doesn't really offer this interface, and I'm really not convinced by the usability of e.g. OwnCloud. Any solutions I might be missing? Or is this something I have to build myself?
Please do let us know about the things you miss – on our issue tracker at http://github.com/owncloud/core/issues
And if you use IRC you’re welcome in #owncloud-dev (freenode).
And be sure that we have a lot of improvements lined up for ownCloud 7 which will come out in a few weeks.
EDIT: Also, if you have specific feedback on ownCloud’s usability and design let me know, I work on that.