Mongo is the most popular NoSql db.
Couchbase is not related to CouchDB.
NoSql is not a particularly useful term since it only tells you what it isn't.
Couchbase has built-in managed Cache and does key-value store, but mostly used for a document database.
Couchbase is open-source and has free licenses.
Couchbase scales easily.
Instead of SQL uses N1q1 ("Nickel") which supports SQL-like syntax. Supports indexes.
Supports Upsert() operation "Update"+"Insert"
Nickel tutorial at http://query.pub.couchbase.com/tutorial/
linq2Couchbase is a open source project to merge linq and Couchbase.
var query = from x in bucket.Query