What is needed to host website which uses Cassandra?
Are there any available options on market right now?
What other NoSQL hosting plans do you recommend?
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I don't know any existing DaaS (Database as a Service) company that provides support for Cassandra. There's also a similar question on StackOverflow, and the answer is always the same: setup a VPS and install Cassandra on your server.
Conversely, there are several alternatives for other NoSQL and/or Document Oriented Databases such as CouchDB, MongoDB and Redis.