That's the beauty of the NoSQL landscape (not a fan of the term). Each db has a specific quality, and while a general SQL db can be moulded to meet a certain need, there may already a db solution that is perfect for it.
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.