CB Insights (www.cbinsights.com/newsletter)
We have 289,000 on the newsletter which represents a large group of VC, tech M&A, corporate strategy and startup folks interested in data-driven discussion of technology trends. It's the primary way we sell subscriptions to our SaaS platform.
We messed around with ads in the newsletter but they don't monetize nearly as well as the "house ads" to our data/product or to our events.
It's our company's golden goose.