The best open source alternative to BuiltWith is Blacklight Privacy Inspector. If that doesn't suit you, our users have ranked more than 25 alternatives to BuiltWith and three of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to BuiltWith are Ingredients.work and HTTP Observatory.
BuiltWith alternatives are mainly Website Audit Tools, but if you're looking for
Website Monitoring Tools or Privacy Tools you can filter on that. Other popular filters include
Open Source. These are just examples - use the filter bar below to find more specific alternatives to BuiltWith.