We used BuiltWith Pro (our own tool) to find people who signed up for our service that were customers of HubSpot - then reach out to them in a meaningful way and introduce to them the HubSpot Sync functionality we recently added to BuiltWith.
We thought this would be a great way to use our own product in a way that customers also can. It's called Eating Your Own Dog Food - a term apparently thought up by Microsoft in the 1980's.
Eating our own dog food was super successful! Attribution: WiredWitch
Comparison of our Dog Food mail campaign to our other email campaigns.