SpiderOak looks great. The only issue is that, since the client is not Open Source (or is it?), you have to trust what it does and you cannot verify it in the code.
I naively attempted to roll my own mash-up for this purpose (https://github.com/clebio/duplicitySync) and was not successful. The combination of local encryption, incremental backup, and _synchronization across multiple computers_ was the breaking point.