While MongoDB can be managed well from the command line, it is often nice to have a GUI app to get an overview of a collection, server status, or to easily interact with other OS X apps.
I started using MongoHub a few years ago, but the original author hasn't had any commits since about that time. Having started working with Meteor.js recently, I needed to work with MongoDB more and wanted a current OS X MongoDB application. Luckily, Fotonauts forked the original code and are actively maintaining it here (Thanks to Thomas Lattimore for pointing this out to me!). This is an example of open source working right.
MongoHub is pretty simple to use. Just setup a connection to your MongoDB instance and open it up. From there you can browse to server stats, collection stats, and perform all of the queries you need. A nice feature for some is that you can import and export MySQL data. I have tested importing a Drupal watchdog table and it does seem to work well.
Aside from MongoHub functioning well, I really appreciate that it is a native Mac app. I can't stand Mac Java apps! I want my Mac apps to behave as expected.
The fotonauts folks are requesting pull requests. If you find a bug and fix it in your fork, please pull request to them so it can be included in the active repo.
One last thing, you should still learn to access and query MongoDB from the command line. It is great to have both CLI and GUI tools accessible.