Also Mongo Machine: http://mongomachine.com/
There's https://mongohq.com
And there's a Heroku add-on that supports it as well.
Re:MongoDB 1.6 stable released
by thibaut_barrere on Thursday August 05, 2010 @04:13PM
(#1579194)
You can also use MongoHQ directly if you want, from Heroku.
I described the steps here:
http://blog.logeek.fr/2010/6/29/sinatra-heroku-mongodb-mongo...
Hmm, now all there's left is an online hosting solution like Couch.io