I love Backblaze. It's employee owned, not VC backed. They publish everything about how they do it.
Granted the comparison is not fair, because S3 is way more performant at scale and for concurrent access... Backblaze is designed for more or less cold storage. But for backups, that's exactly what you want.
I'm about to build an Unraid server myself -- 10TB for under $1000 using commodity hardware including storage, expandable to 30 TB pretty easily. So I guess I'm a bit of a storage nerd. I believe all things should be stored forever. With companies like Backblaze, it looks like it may well be within humanity's grasp.