Contact Tracker provides ability to add extra information in contact email which is built in Drupal core.
The module itself doesn't add any information into the email. It provides two hooks so that other modules can add their information by implementing those hooks. One hook is used for registering - let Contact Tracker module know another module wants to add info. The other hook is used for processing data. Contact Tracker module calls this hook to get info and put into email.
In Drupal contact-form add/edit form, This module provides a configuration section. Site administrator can choose extra information
they want to add for certain contact category. The options are get from "registry hook" mentioned above. Admin can also choose to put extra info above or below email body.
This module has a settings page where site administrator can set separator above/below extra info in email. This makes the extra info more obvious in the email.
Two sub-modules are given within this module, which implement two hooks of Contact Tracker module and can be used as example. They are also very useful. One adds HTTP referrer info and the other one adds user's session info into contact email.