HaveIBeenPwned is a website that allows users to check if their personal data has been compromised by data breaches. For a penetration tester preparing a credential stuffing attack, HaveIBeenPwned can provide valuable information about which accounts and passwords have been exposed, making them more likely targets for successful credential stuffing. This passive information gathering tool can help in identifying the most relevant credentials without actively probing the target's systems. The other tools listed (Shodan, BeEF, Maltego) serve different purposes, such as device and service enumeration, client-side exploitation, and information gathering through different means, respectively.