From a very superficial view - this is my understanding of some NoSQL alternatives.
CouchDB – stores JS-documents. Light, good replication/synchronization, sometime used for offline storage and mobile devices. Can be used in low-write intensity web-sites. Implemented in Erlang.
MongoDB – stores JS-documents. Fast DB implemented in C++. Can be used in relatively high write-intensity web-sites. Supports multi-master mode. Supports basic sql-like queries. Recently added auto-sharding.
Cassandra – full blown implementation of Amazon’s Dynamo combined with column-based storage. Relatively new product designed for massive scale. Of course, complexity and “raw”-ness are potential downsides. Used by Facebook.
Voldemort. Massively scalabled, replicated, redundant DB with pluggable storage engines. Also based on Amazon’s Dynamo. Provides most sophisticated features and scale at expense of complexity. Used by Linked-in.
Hadoop – can store huge amounts of data that is accessed through map/reduce functions. Frequently used in analytics and data-mining.