ExpressionEngine 3, ExpressionEngine 4
Publisher Lite is the workflow only version of Publisher. It does not support multiple languages, but you get all the other benefits of Publisher.
ExpressionEngine 1.x, ExpressionEngine 2
Project Cerberus is an antispam webservice specifically targeting ExpressionEngine member profile spam.
Purge is an extension for that sends a purge request to the Varnish caching proxy upon entry submission/deletion. This allows you to keep objects in the cache longer while still being able to keep your site up-to-date.
A simple accessory that turns the 'Pages URI' field on the publish page into an autocomplete text input.
Helps users generate a Pages Module URI by selecting a parent page and automatically pulling the URL Title from the entry.