Stream is a free WordPress plugin that allows you to remain informed of what goes on with your website. If you have multiple authors or contributors to your site, it will be even more powerful. The activity log can give you a picture of what has been going on with your site such as plugins being enabled, posts being created or updated, and attempts to login.
You can set what types of accounts have their activity recorded and what types of accounts have access to view the resulting activity log. You can also choose what WordPress components are logged. You can choose from:
By default, Stream keeps records for 90 days but you can increase or decrease that setting.
Stream is a very valuable plugin, particularly if multiple people have access to the administrative back-end of your WordPress site. It can give you insight to security problems or assist troubleshooting a problem. The auto-changelog is handy and does not appear to put much load on the server. You can visit Stream from its own menu entry below Dashboard or with the Dashboard widget.
Stream requires PHP 5.3 or later. You can install Stream from WordPress.org for your self-hosted site and look for more extensions coming soon like Stream Cherry-Pick, which allows administrators to delete individual logs.