There are a lot, so here are the most important ones.
hsd_edd_app_early_html If you want to create your own output for the EDD integration use this filter to create your own HTML response.
hsd_edd_app_html This filter could be used if you want to manipulate the HTML response for the EDD integration.
hsd_create_conversation Is an action fired after a conversation is created.
hsd_create_conversation_fields Filter the fields sent to the Help Scout API to create a conversation.
hsd_create_thread Is an action fired after a thread/reply is created.
hsd_create_thread_fields Filter the fields sent to the Help Scout API to create a thread/reply.
hsd_get_conversations_by_user Filters the API response for conversations based on a user.
hsd_get_conversation Filters the API response for a conversation based on a conversation id.