Here is a list of all namespaces with brief descriptions:

actionlib
actionlib_msgs
client
diagnostic_msgs
geometry_msgs
kitt_debug_msgs
kitt_nav_msgs
kitt_sensor_msgs
nav_msgs
ros
roscpp
rosgraph_msgs
rosserial_arduino
rosserial_msgs
sensor_msgs
shape_msgs
std_msgs
stereo_msgs
tf
tf2_msgs
topic_tools
trajectory_msgs
visualization_msgs