OpenKeyval is open-source, so you can host it yourself internally and add whatever access limitations you like.
CouchDB is basically OpenKeyval on steroids. It provides a RESTful JSON API than can be accessed from any environment that allows HTTP requests (i.e., JavaScript) but you'll have to host it yourself since there aren't any public-facing CouchDB servers (that I know of).
And since you're looking for a list:
- Amazon SimpleDB
- Hypertable
- Cassandra
- Hadoop
- Ringo
- ThruDB
- CouchDB
- HBase
