Drupal 6 supports [OpenID][oid] out of the box.
If kayako provides an API for user authentication, you can also implements an external authentication module for Drupal. This module will have to implements [hook_form_alter()][1] to edit the [user_login][2] and [user_login_block][3] forms and replace [user_login_authenticate_validate()][4] in their `$form['#validate']` by the module's own *validation* callback that should authenticate the user.
[oid]:
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[1]:
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[2]:
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[3]:
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[4]:
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