A prompt for the mongo shell that displays the user you're logged in as and the database you're pointed at.

.mongrc.js

/**
Set the mongo shell prompt to display [user name]@[database name].  If the user or database name cannot
be captured, then display "<none>" instead.
*/
var prompt = function() {
        var user = db.runCommand({connectionStatus : 1}).authInfo.authenticatedUsers[0].user || "<none>";
        var dbLabel = db.getName() || "<none>";

        return user + "@" + dbLabel + ">";
}