Gaius is a great multipurpose moderation bot. In /r/Akame ga Kill we use it for a variety of things.
The autoban feature on users with blank avatars, and on users via the global ban system has prevented potentially nefarious accounts from joining our server on many occasions. Other features like deleting links, chainmail spam, etc also work towards keeping the server clean.
The logging system is useful for keeping records in a dedicated streamlined channel & we also make use of the say command where applicable.