http://schemafreedb.com/demo.html not a doc store but allows the storing of semi-structured data with a sql like query language with a json over http api.
[ {"query": { "_queryId":"office-query", "sfql": "SELECT $phone.oid, $s:phone.name, $s:phone.value WHERE $s:phone.name='office'" }}, {"query": { "_queryId":"mobile-query", "sfql": "SELECT $phone.oid, $s:phone.name, $s:phone.value WHERE $s:phone.name='mobile'" }} ]
[ { "_queryId": "office-query", "cmdname": "query", "data": [ { "phone_oid": "3", "s:phone_name": "office", "s:phone_value": "111-112-1113" }, { "phone_oid": "7", "s:phone_name": "office", "s:phone_value": "111-112-1113" } ] }, { "_queryId": "mobile-query", "cmdname": "query", "data": [ { "phone_oid": "4", "s:phone_name": "mobile", "s:phone_value": "222:223:2224" }, { "phone_oid": "8", "s:phone_name": "mobile", "s:phone_value": "222:223:2224" } ] } ]