mongostat
The mongostat utility provides a quick overview of the status of a currently running mongod or mongos instance. mongostat is functionally similar to the UNIX/Linux file system utility vmstat, but provides data regarding mongod and mongos instances.

mongotop
mongotop provides a method to track the amount of time a MongoDB instance spends reading and writing data. mongotop provides statistics on a per-collection level. By default, mongotop returns values every second.

mongoreplay
mongoreplay is a traffic capture and replay tool for MongoDB that you can use to inspect and record commands sent to a MongoDB instance, and then replay those commands back onto another host at a later time.

mongo
mongo is an interactive JavaScript shell interface to MongoDB, which provides a powerful interface for systems administrators as well as a way for developers to test queries and operations directly with the database. mongo also provides a fully functional JavaScript environment for use with a MongoDB. This document addresses the basic invocation of the mongo shell and an overview of its usage.

profiler
The database profiler collects fine grained data about MongoDB write operations, cursors, database commands on a running mongod instance. You can enable profiling on a per-database or per-instance basis. The profiling level is also configurable when enabling profiling. The profiler is off by default.

compass
The GUI for MongoDB. Visually explore your data. Run ad hoc queries in seconds. Interact with your data with full CRUD functionality. View and optimize your query performance. Available on Linux, Mac, or Windows. Compass empowers you to make smarter decisions about indexing, document validation, and more.

mtools
mtools is a collection of helper scripts to parse, filter, and visualize MongoDB log files (mongod, mongos). mtools also includes mlaunch, a utility to quickly set up complex MongoDB test environments on a local machine.