Couchbase was a merger between NorthScale and CouchOne. The CouchOne founding team were all CouchdDB project founders: Damien Katz, J Chris Anderson, and Jan Lehnardt. NorthScale was founded by Steve Yen, who was an Entrepreneur in Residence at Accel, and Dustin Sallings, who was a committer on the Memcached project.
The first product that NorthScale released was called Membase, and the point of it was to help companies run large memcached clusters without having to think very hard. Couchbase later evolved into a hybrid of Membase and CouchDB that can speak the memcached protocol but also has some of the JSON document database properties that CouchDB has.