Personally I'm enjoying playing with Skylable SX server (and its libres3 s3 interface) - I'll be honest, I never really gave Swift enough of a chance. I've yet to get my head round it quickly enough so far. Found this one while researching other things
"Skylable was founded in 2012 by Tomasz Kojm, Luca Gibelli, Edwin Török and Alberto Wu, best known for their success with ClamAV, the leading GPL anti-virus for UNIX running on millions of servers and protecting billions of users."
"With SX Cluster you aggregate the disk space available on multiple servers and merge it into a single storage system. The cluster makes sure that your data is always replicated over multiple nodes (the exact number of copies is defined by the sysadmin) and synchronized. Additionally Sx has built-in support for deduplication, client-side encryption, on-the-fly compression and much more"
Very responsive in their IRC channel on Freenode too. I know I sound like a walking advert reading this comment back but I have no affiliation other than end user
As mentioned elsewhere on this thread, you'd use Swift (and other object storage systems) to store larger pieces of unstructured binary data (eg. images/videos/backups). The easiest way to do that in Cassandra without rolling a whole bunch of new code for yourself would be to store these things as gigantic rows in Cassandra, which isn't where it performs well. Typically object storage systems also come with a whole lot of features such as being able to control who views your files, the ability to upload files in piecemeal fashion, etc.
Swift is part of the larger family of OpenStack. Swift should provide (haven't verified this myself) better integration with all of the other OpenStack components (Keystone, Cinder, Neutron, etc.).
Swift can integrate with OpenStack, but you can very easily run it stand alone, and many do. It has erasure coding, geo replication, and its a fairly well baked product.
http://docs.openstack.org/developer/swift/
It can be run as a stand alone app so it's generally pretty simple and can include support from https://swiftstack.com/.