VeraCrypt

VeraCrypt is a free, open-source disk encryption software for creating encrypted virtual disks or encrypting entire partitions. While not a cloud service itself, encrypted VeraCrypt containers can be stored in any cloud storage for secure, zero-knowledge sync. It's considered highly secure, used by advanced users and organizations. The process is more manual than integrated services like Cryptomator.

It represents the DIY, maximum-control end of secure cloud storage, allowing users to leverage any cloud as mere encrypted blob storage. It's included for its role as a foundational security tool for technically adept individuals.