Dropbox is an easy to use freemium cloud storage solution. It has a client sync utility for multiple platforms.

MEGA is the cloud-based file hosting solution.

Copy.com is a discontinued cloud storage service that offered near-unlimited space and huge bonuses for referrals.

Pulse is a free (as in freedom), secure, and distributed file synchronisation engine.

SpiderOak is a cloud-based backup and restore solution to protect against data loss.

Resilient, fast and scalable file sync software for enterprises and individuals.

Koofr offers safe EU based cloud storage with 2 GB free cloud storage space for life and options to connect multiple cloud accounts.