From time to time I publish the details a project which I am tinkering with.
GDPR Pseudonymization Tool
This is a simple online tool that allows you to store personal data that has been pseudo anonymized via a one-way hash. By including other privileged but non-personal data in the search function, you are able to confirm if a given example personal data is present in the database or not.
This was developed for a charity who wanted to be able to maintain a record of engagements with third parties but not allow anyone to query their database in plaintext.