CouchDB requires explicit indexes, but they can be arbitrarily complex, which is the right approach for a database focussed on scalability. They are also fun to write (javascript + map/reduce).

It appears that JavaScriptDB just maintains indexes of everything, until you have over 2000 objects of a class and then it only updates the indexes of fields when you search on them. This saves you the trouble of initially specifying indexes, but by the time your DB gets big enough that they become important, you'll have to manage them explicitly anyway, and you don't get anything besides plain old lexical sort. Persevere looks great for rapidly developing small projects, but not particularly scalable. The automatic index thing might be handy for a multi-faceted search, which is a pain to do in CouchDB.