https://tineye.com/ - Tineye reverse image search
https://yandex.com/images/ - Yandex provides a powerful reverse image search. I have personally used Yandex successfully in numerous romance scam investigations.
https://pimeyes.com/ - PinEyes is a reverse image search and facial recognition search engine. It can be used to search for individuals' faces as well as specific images.
If you know of any other useful reverse image tools then please share your knowledge by posting a reply to this thread.