10gen has some seriously awesome (free) monitoring software: MMS.
You’re supposed to add your machines to a group, and use a different group for each logical grouping of machines. For instance, one group for one production cluster and another group for a different cluster. But how do you add a new group!?
I admit the obvious answer eluded me. You do it from the Admins page. Once there you’ll see a little “Add New Group” link:
Once you do that you can switch among your groups with the select list in the upper-left corner.