I've always been a fan of Amazon Web Services (that's why I built Sendy!). Arq (Cloud Backup for Mac) now supports Amazon Glacier, a low cost storage service (even cheaper than S3) for infrequent data retrievals. Which makes it good for backups.
With Glacier you can store 100GB for just $1/month! Or store a terabyte for just $10/month!
If you're looking for a low cost backup service from a provider you can trust, don't overlook Arq + Amazon Glacier.