Simple History is a WordPress plugin that logs various things that occur in WordPress and then presents those events in a very nice GUI.

Download from WordPress.org:
https://wordpress.org/plugins/simple-history/

This screenshot show the log view + it also shows the filter function in use: the log only shows event that are of type post and pages and media (i.e. images & other uploads), and only events initiated by a specific user.

Simple History uses the log levels specified in the PHP PSR-3 standard.

Each logged event can include useful rich formatted extra information. For example: a plugin install can contain author info and a the url to the plugin, and an uploaded image can contain a thumbnail of the image.

Developers can easily log their own things using a simple API:

<?php
// Add events to the log
SimpleLogger()->info("This is a message sent to the log");

// Add events of different severity
SimpleLogger()->info("User admin edited page 'About our company'");
SimpleLogger()->warning("User 'Jessie' deleted user 'Kim'");
SimpleLogger()->debug("Ok, cron job is running!");

You will find more examples in the examples.php file.