There are many Dropbox alternatives using their own protocols (Owncloud, Seafile, etc) or a de-facto protocol (SparkleShare, etc).
How about alternatives using a rock-solid standard protocol?
- For each protocol, list at least 1 server and 1 client, preferably open source, any operating system
- The protocol must have been validated by either ISO, OASIS, W3C, IETF(RFC), ECMA, IEC
- A protocol is not made standard just because it uses HTTP/XML/SSH/etc
- File synchronization, conflict handling
- Bandwidth as low as possible