Ask HN: How did you acquire your first 100 users?: https://news.ycombinator.com/item?id=14191161
Ask HN: How did you grow from 100 to 1,000 users?: https://news.ycombinator.com/item?id=14227063
Some "free" ones: PR would be a good one, track any relevant industry publications, follow #journorequest on Twitter [0], and reach out to journos writing articles about your industry/area and offer comment.
Reach out to some industry blogs, or the blogs of companies in non-competing verticals to write some value-add guest content. Builds a network, and if you find somewhere where you'd be an additive to another brand (e.g. you provide an email helpdesk solution, can write an article for an ESP about best practices for outbound email content etc...)
Make sure you capture interest on your site, landing page, blog email capture, reaching out to cold leads and asking them if they would be interested in hearing more (make sure you're explicit for their permission). Then keep them on a drip email list outlining the problem you solve, how it fits into the industry, some kind of hook relevant to each user, and then an offer to talk them through getting set up, or hear about what blockers they'd have with implementing your solution. Sell after a build up.
None of this is quick, organic acquisition will compound, but don't expect results with free channels straight away.
Next thing is social visibility (for example Facebook, Twitter, blogs etc.). Especially inside look for product development and new upcoming features give the users something to wait for. Also referral programs (discount or some other benefit for people who get friends and family join to use the product) are proven to be very working concept.
Also one thing to consider is to allow people use the product for free for example for 1 month and after that either automatic or manual contact to continue the usage and start paying.
The whole thing depends a lot what kind of product you are talking about. Applications have their own channels where you can get users from for example app reviews done by some bloggers on the internet.
1. Posted landing page in a slack chat (first ~10 users)
2. Posted on the indiehackers forum (+30 users)
3. Got mentioned in the indiehackers weekly newsletter (+150)
4. Posted to hacker news (+?)
5. Posted to product hunt (did not go so great)(+?)
6. Started posting weekly metrics to HN - made the frontpage (+1500-2000)