As part of setting up an application that used twitter/google/github for authentication, I began playing with some of the cloud providers. I was going to use Heroku, but to use mongodb apparently required a credit card, even though there are not supposed to be any charges. Now, they didn't say it would cost anything, but I did not want to run the risk of having it start charging anyway if I somehow blew through the free tier limits.
So, I then saw that RedHat's OpenShift platform also offered a test drive of mongo in your node app, and it did not require a credit card for that. And sure enough, using just my email address, I got a little app using mongo to work, with user authentication via twitter (OAuth) using passport.js. This little app is by Madhusudhan Srinivasa and is on github.