It has a few rough edges, but ownCloud is generally a good way to store and sync files via your own private server. It also covers contacts and calendars, making it more useful than just a simple file store, but the upcoming version 6 is going to be a major leap forward. Its biggest new feature is document collaboration: users can work together on rich text documents, with highlighting to show who has edited what.
It’s based on the ODT file format (as used by Open/LibreOffice), and while it’s still in the early stages of development, it could eventually give Google Docs a run for its money. Check out the video from the ownCloud team.