Tarsnap (haven't used it - too complex and pricey for my use case; and not sure if it's open source). FWIW, Tarsnap is not under an open source license, but yo

Tarsnap (haven't used it - too complex and pricey for my use case; and not sure if it's open source). FWIW, Tarsnap is not under an open source license, but you do get the client source code (and you're encouraged to audit it before compiling).