Python has many built in modules, but probably some of the most used ones are random (provides pseudo-random stuff) and math (allows you to do more complex math operations, like sines and square roots). Each Python module has something that it is used for, and you can check out all of them at the Python Documentation: https://docs.python.org/3/library/
Every module from the standart library is essential by definition. The most commonly used depend on the application area. As for me, I often use re, math, random, os, sys, html, xml.etree, urllib (3rd party urllib2 to be faithful, but it's almost the same), unittest.