API per functionality

For a list of common ROS repositories, see the ROS GitHub Organizations.

ROS supports several Client Libraries, though the main supported libraries are C++ roscpp and rospy. There are also numerous ROS stacks and packages that provide higher-level functionality. Support for higher-level functionality in various languages is summarized below.

APIs

ROS: functionality available via ROS topics and services
C++: functionality available in C++ libraries
Python: functionality available in Python modules/packages

API	ROS	C++	Python
ROS	ROS	roscpp	rospy
Basic Datatypes	common_msgs	common_msgs	common_msgs
Manipulating message streams	topic_tools	message_filters	message_filters
Drivers	joystick_drivers, camera_drivers, laser_drivers, audio_common (aka sound_drivers), imu_drivers	joystick_drivers, camera_drivers, laser_drivers, audio_common, imu_drivers
Driver Implementation	driver_common	driver_common
Filtering data		filters
3D processing	laser_pipeline, perception_pcl	laser_pipeline, perception_pcl
Image processing		image_common, image_pipeline, vision_opencv	vision_opencv
Transforms/Coordinates		tf, tf_conversions, robot_state_publisher	tf, tf_conversions
Actions	actionlib	actionlib	actionlib
Executive/Task Manager	executive_smach		executive_smach
Navigation	navigation	via actionlib	via actionlib
Simulation (2D)	simulator_stage	simulator_stage
Simulation (3D)	simulator_gazebo	simulator_gazebo
Robot Model		robot_model
Realtime Controllers	pr2_controller_manager	pr2_controller_interface, realtime_tools
Motion planning (arms)	ompl, chomp_motion_planner, sbpl	actionlib through move_arm	actionlib through move_arm
Humanoid walk	walk_msgs	walk_interfaces

Packages in Layered Layout

More slide-friendly layout of ROS packages in a layered manner.

Note:

Packages are subjectively chosen from "Browsing packages for indigo" page. Original author has no intention to omit any packages due to his personal reason.
You can zoom this diagram, or edit on Google Drive.

Improvements/corrections for the package grouping diagram

Particularly following may require improvements/corrections (please remove the lines when addressed):

There should be so many more notable packages in the application layer.
Maybe too much focus put on navigation and MoveIt!?
Some pkgs in "Comm layer" are not really communication purpose.
RViz and rqt may better be drawn larger considering how well they are used?
Cosmetics (coloring?)