mongostat

Fields:
   insert       - # of inserts per second (* means replicated op)
   query        - # of queries per second
   update       - # of updates per second
   delete       - # of deletes per second
   getmore      - # of get mores (cursor batch) per second
   command      - # of commands per second (on a slave, it's local|replicated)
   flushes      - # of fsync flushes per second
   mapped       - amount of data mmaped (total data size) megabytes
   vsize        - virtual size of process in megabytes
   res          - resident size of process in megabytes
   faults       - # of pages faults/sec (linux only)
   locked       - percent of time in global write lock
   idx miss     - percent of btree page misses (sampled)
   qr | qw      - queue lengths for clients waiting (read|write)
   ar | aw      - active clients (read|write)
   netIn        - network traffic in - bits 
   netOut       - network traffic out - bits
   conn         - number of open connections
   set          - replica set name 
   repl         - replication type 
                    M    - master
                    SEC  - secondary 
                    REC  - recovering
                    UNK  - unknown
                    SLV  - slave
                    RTR  - router

mongostat --host a,b,c

mongostat --discover (--host optional)