I've entirely replaced my usage of Dropbox within the past two years with Syncthing. It works great if you're on Android. Doesn't have an iOS client though.

Nice thing about Syncthing is you can entirely configure what you are syncing and how. I literally press one button on my phone, it creates a backup, and that backup is sent to my server.

I would go as far to say that Android's killer app is Syncthing. Yes, I'm putting this on the same level as iMessage for iOS. It's that good. If I ever switch back to iPhone I will really miss it.