I know of a number of different cloud storage vendors with all sorts of different features. I'm looking for a way to sync files between different machines without the service provider being able to read them: only my machines should have the keys. Does the provider have clients for major desktop operating systems (I run Debian mostly, but Windows and Mac support are also interesting); how about mobile clients? What sort of crypto do they use, and how are keys managed? Is the client free software, or source available? Is there any way to share files with other users; what about people without accounts?
Use encfs with your favorite cloud storage provider. encfs is available for most major Linux distributions, and you can download BoxCryptor for easy-to-use Mac, Windows, and mobile cliens that are compatible with encfs. encfs is open-source and free, but BoxCryptor is not.
With encfs, the encryption/decryption will happen on your client machines, so the cloud vendor will not have the ability to read your data. If you ever decide to switch storage vendors in the future, you can just move the encfs-encrypted data to the new provider.