Drop.io lets you put documents, files, web links and other data into a "drop" which you can then share with others and collaborate on. The service is limited to 100MB for free drops, with a premium account giving access to more storage space as well as secure file management tools, and other benefits.
DropBox simplifies the task of backing up files and syncing them between multiple computers and other storage devices. The service will automatically replicate any file stored within it on every computer that you install the DropBox client on, as well as keeping a copy online. You can also selectively share folders and invite others to view them. The DropBox client works with Windows, Mac, Linux and iPhone. The free level gives 2GB of storage space, which can be increased to 100GB by paying a monthly fee.
JungleDisk lets you store files and automatically back up your data securely to Rackspace Cloud and Amazon's S3 service. Their pricing model is very simple: a small amount for every GB stored each month, with no minimum or maximum charge. Backups can be automated to run in the background, and you can treat your JungleDisk storage just like any other hard drive, and drag and drop files into it.
OrbitFiles offers several online file storage plans, including 6GB of free storage in the most basic plan (subject to various limitations, and a 10MB maximum file size). Data stored under the free plan will be deleted if there has been no user login for 45 days. Their paid service provides automated backup software that will run unattended, greatly enhanced storage space and the ability to upload files of up to 1GB in size at the highest service level.
SkyDrive offers 25GB of free online storage space as part of the Windows Live family of tools. You can password-protect your files or choose to share them publicly, and you can drag files into your online folder just like on your PC.