For our shared files, we use SparkleShare. It provides an experience very similar to Dropbox: you have a SparkleShare folder that is synced up with the service, and in turn any other users who are also linked up to it. Once its setup, it is as easy to use as Dropbox, but setting it up takes a bit more work than Dropbox. It builds upon two software packages well known for security and reliability: git and ssh, and works with Tor Hidden Services. It runs on Windows, Mac OS X, and GNU/Linux, and there is an Android client in the works.