I find a good product needs some, but minimal marketing. Customers will recommend it to friends if it's a good fit and it'll continue to grow that way, and obviously try to get some SEO. To get started, I'll regularly post on Reddit, HN, Slack, Facebook, etc. (typically providing some insightful comment, using one of my applications) and I'll get signups. One of my particular posts recently got me 50 sign ups for my website, to put it into perspective.

Those people are also my target customers though, so it makes it easy.

When "marketing" to companies, a cold call, email, or intro from a friend usually can pique their interest. Then you just need a really good demo / app. Even then, they likely wont bite; hence I'm moving away from that business and mostly to consumer apps.