Checking dependents on GitHub is the best way to see a list of open source projects using synpress:
https://github.com/Synthetixio/synpress/network/dependents?dependents_after=MzU5MjAxMjUyNzY&package_id=UGFja2FnZS0xNjIwNjMzMjE1

You can find many big projects there if you focus on amount of stars given for a repo on page above.