This is a listing and brief explanation of the various helper methods available through autotest_lib.client.*. It is in no way comprehensive, and is only meant as an introduction to the topic:
auth_server - auth
dns_server - auth
login - wait_for_(browser, window manager, ownership)
autotemp - autotest temp dir creation helpers
base_utils - ‘run’ and other system level helpers
error - different error classes
base_utils - linux sysadmin helper: grep, disk, cpu, environ
site_utils - systems level helper: ping hosts, board type.
test - main test class
unit_test - main unit_test class
If you have come this far, you may also be interested in reading the autotest client tests codelab.