But the differences between "NoSQL" databases are much bigger than it ever was between one SQL database and another. This means that it is a bigger responsibility on software architects to choose the appropriate one for a project right at the beginning.
In this light, here is a comparison of Cassandra, Mongodb, CouchDB, Redis, Riak and HBase:
CouchDB
Written in: Erlang
Main point: DB consistency, ease of use
License: Apache
Protocol: HTTP/REST
Bi-directional (!) replication,
continuous or ad-hoc,
with conflict detection,
thus, master-master replication. (!)
MVCC - write operations do not block reads
Previous versions of documents are available
Crash-only (reliable) design
Needs compacting from time to time
Views: embedded map/reduce
Formatting views: lists & shows
Server-side document validation possible
Authentication possible
Real-time updates via _changes (!)
Attachment handling
thus, CouchApps (standalone js apps)
jQuery library included
Best used: For accumulating, occasionally changing data, on which pre-defined queries are to be run. Places where versioning is important.
For example: CRM, CMS systems. Master-master replication is an especially interesting feature, allowing easy multi-site deployments.
MongoDB
Written in: C++
Main point: Retains some friendly properties of SQL. (Query, index)
License: AGPL (Drivers: Apache)
Protocol: Custom, binary (BSON)
Master/slave replication
Queries are javascript expressions
Run arbitrary javascript functions server-side
Better update-in-place than CouchDB
Sharding built-in
Uses memory mapped files for data storage
Performance over features
After crash, it needs to repair tables
Best used: If you need dynamic queries. If you prefer to define indexes, not map/reduce functions. If you need good performance on a big DB. If you wanted CouchDB, but your data changes too much, filling up disks.
For example: For all things that you would do with MySQL or PostgreSQL, but having predefined columns really holds you back.
Read more: Kristóf Kovács
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