Mongo Admin Tools
To get information on the state of a running mongod process, some external tools are available.
The only one bundled with MongoDB is the HTTP console, which provides a simple web interface listing information of interest to administrators. It can be accessed at the port with numeric value 1000 more than the configured mongod port: that is, for the mongod listening on port 27017, the console can be accessed at http://localhost:28018 in a web browser. Some of these information can be accessed by launching mongotop or running the db.currentOp() command in the shell.
By launching mongod with the additional --rest parameter, two more services are available on the same port. One is the Simple REST interface, which only supports data retrieval: no create, update or delete but it allows to issue any kind of query using URLs and returns the result as strict JSON. It can be found at http://localhost:28018/dbName/ where dbName is the name of the database to query.
Finally, there's the Replica Set Admin UI. For some reason, you also need to have ran your server with:
mongod --rest someOtheStuffHere
It can be found at http://localhost:28017/_replSet or by just following the link from the root page. It has additional information on the status of the replica sets, if any set is available. Some of these infp can be accessed with the db.printReplicationInfo() shell command.