Syncthing is great, and I use it at home. For a robust multi-user alternative to Dropbox (or *cloud), I can also recommend Seafile. I replaced Dropbox with a self-hosted version of Seafile and have never looked back. Also, for a fantastic mail server solution with a great webmail client, look at Axigen. Their free version is more than enough for a personal server, and you can use Amazon SES for outbound mail to avoid reputation issues. I host mine at Linode and love it. If you have a business need or are larger than the limits of the free version, their license costs are quite reasonable.