Out of the database systems I surveyed my favourite one is Couchdb.

Compared to mongoDB, or all the SQL variants, it’s just plug and play.  After installing to the server, I don’t need to write any server-side code to hook up data from client side apps.  I can simply load and save data directly from client-side javascript code using AJAX GET/PUT/POST methods.  The documentation is great, includes a free O’Reilly ebook, and the coding philosophy encourages one to simply relax.