My own plugins, all with a free version that's hosted on WordPress.org. The free version of my most popular product has hundreds of thousands of users and is what brings in the bulk of my customers.

No paid advertising. I tried but it never really got to a point where it was worth it.

Sorry for being a little vague, I'd rather not state my revenue and product together. And sine I've already mentioned my revenue, I won't mention the product. ;)