All I want is to encryptedly back up some directories on my home server, but nothing* really does that. I use SpiderOak at the moment, but it's not OSS.
No OSS linux client, but we do have a closed source one. CrashPlan Does work headless[1], but note that that setup out of the scope for our support team.
I can't say one way or another if the client is OSS, but I do know you can run it headless, I've done it before.
IIRC, you need to tweak a non-headless client to direct it to the headless instance (some config file to point to the server vs localhost) and everything works from there.
Look into Crashplan. Client-side encryption, unlimited versioning, never deletes files, and only $6/month for truly unlimited backup.