Depending on your needs, you could try Syncthing [1].
IMO the major functional differences between it and Dropbox are:
* You have to provide the servers that Dropbox provides. I.E. you have to provide the level of availability that you want. [2]
* FOSS licensed.
* Because your data is only ever stored on your machines, you are only limited by your own storage and bandwidth.
[2] This requirement has actually loosened to some degree because the latest version of Syncthing introduced relay servers. So you still need to provide available storage servers, but you can use relays to overcome partitions in your network.