The Aquarium Wiki:CheckUser

The CheckUser extension allows people who have been granted this right to check account information and other data.

If you are a checkuser, then you can view a list of IP addresses used by every user account, as well as a list of all accounts that have used a certain IP address.

Because of the tool's nature, you have to be 18 years or older of age to use it, provided that you're mature enough to use this tool without causing trouble.