At work, we use Dropbox to share a small number of files between two offices, mainly schedules and logs to keep track of stuff.
For sharing bulky files between multiple offices, we use FTP. Although more complicated than a folder sync service, it’s cheap. Again, we mainly use it for non-confidential files such as product photos, editable catalogues and product manuals.
For personal use, I occasionally use Google Drive.
One “cloud” service I may experiment with is Usenet. While not free, it would be an extremely cheap service for bulk backing up of files (i.e. 100s Gigabytes.) Due to the way Usenet works, if your Usenet provider goes out of business, your data is already distributed with other Usenet providers around the globe. There’s no storage charges either regardless of how much is uploaded. The obvious catch is that your data is publicly available to download (if one figures out the articles to download), so everything needs to be securely encrypted before uploading. Other drawbacks include the need to manually upload files and keep track of them (less user-friendly than even FTP, especially with the need to create PAR recovery files, etc.)
One thing I would like is a folder-sync application that works like Dropbox, but which uses your own server instead of theirs.