CouchDB and MongoDB definitely yes. I mentioned that in my book:
- http://use-the-index-luke.com/sql/testing-scalability/response-time-throughput-scaling-horizontal
Here are the respective docs:
- http://guide.couchdb.org/draft/btree.html
- http://www.mongodb.org/display/DOCS/Indexes
NoSQL is, however, too fragmented to give a definite “yes, all NoSQL systems need indexes”, I believe. Most systems require and provide indexes but not at level most SQL databases do. Recently, the Cassandra people were proudly introducing secondary indexes, i.e., more than a single clustered index.
- http://www.datastax.com/dev/blog/whats-new-cassandra-07-secondary-indexes (well, not so recently as I remember)