A Sync may exist between any two or more folders including two folders that exist as part of the same machine (e.g. between a desktop and an external hard-drive). The 5.0 series have introduced the SpiderOak Hive folder. Any items placed inside Hive will automatically upload and then sync to all your other devices (thus effectively replicating the Dropbox folder).
Note: While SpiderOak was formerly a free service, plans currently start at $7/month or $79/year. A 60-day free trial is available.