1. mass_editing: This module provides easily configurable bulk editing options.
2. auth_admin_passkey: This module allows logging in with an extra system administrator password in other user accounts.
3. report_xlsx: This module provides a basic report class to generate xlsx report.
4. auditlog: This module allows the administrator to log user operations performed on data models such as create, read, write, and delete.
5. web_m2x_options: This module modifies “many2one” and “many2manytags” form widgets so as to add some new display control options. The best part is that it supports global option management with ir.config_parameter.
6. web_export_view: This module allows exporting the tree view data to CSV/XLS
Let me know if you guys know of more such essential apps.