Not open source, and I haven't tried it, but I've heard ads for http://www.filetransporter.com http://www.filetransporter.com which may be an interesting soluti

Not open source, and I haven't tried it, but I've heard ads for http://www.filetransporter.com http://www.filetransporter.com which may be an interesting solution for you. They provide discovery, syncing & NAT traversal to a hard drive that you own.