CouchDB is a free and open source #Apache project. It is the predecessor to #Couchbase, which is a commercial incarnation designed by some of the people from the CouchDB project. CouchDB is the database to use if you're looking for open source, fault tolerance and efficient syncing and are also into document oriented databases (read #JSON and #Javascript) with flexible schemas. The fault tolerance and efficient syncing functionality is automagically provided by the underlying implementation layers.
Using CouchDB does require a mindset shift. You need to get the hang of its flat document store (although documents themselves can be nested #JSON documents) and map/reduce features. Once you get used to the way it works it is refreshing and light to use, very powerful, and very efficient. You are given a lot of control and power in the simplest of ways. If you're starting out with CouchDB there is it's #Mango query language (inspired by #MongoDB) that can serve as a good stepping stone into the CouchDB world from other databases. For me I prefer the raw power and control given by directly creating and querying views.
CouchDB ships with the Fauxton Admin UI which I find very simple yet very powerful and fast. You can easily create and replicate databases, create documents and views, and execute #Mango queries from within it.
