And there is already a good "classification" system that you can use to categorize the solutions.

There are Column Stores (Cassandra, HBase), KeyValue Stores (Riak, Voldemort), Document Stores (MongoDB, maybe CouchDB), Graph Databases (Neo4j) and something I usually call Data-structure Stores (Redis).

For Column and KeyValue stores, you can often also say that they're implementations of BigTable or Dynamo.