There's no such thing as a free lunch. However what you can do is find web forums whose topics are related to your product. You need to check if those forums have strict "no advertising" rules which many do. If not, you can probably put a link to your website in your forum signature and then participate in discussions. I've gotten a few leads this way, but it requires a lot of your time. If the subject is one you're interested in anyway then participating in forum discussions can be recreational so it won't seem like work. Otherwise it's mostly a waste of time.
Advertising services on Craigslist is another option, if your product is service based. However there's a lot of challenges involved in that and the site is plagued by spammers so many of your potential customers won't see your ads. Still, it is free.
Beyond that there really isn't anything you can do. If you're a decent writer you can write topical blog posts, or create videos on youtube, which will increase search engine results and can funnel leads back to your website. These things aren't really free because they require a significant investment of time, but perhaps you've got plenty of time to spare.
Advertising on sites like Reddit or Facebook can be pretty cheap but the ROI is dubious.. Advertising on search engines reaches more people but the prices are higher and again the ROI is dubious.
I've been watching businesses on the internet for a long time and it seems like the ones which make the most money are the ones who have the most amazing products. They get traction by word of mouth more than anything, which leads to increased search engine results and mentions in other people's blogs. These are things you can't really control.
Posting your website on a lot of free directories doesn't hurt. Nobody is ever going to see those listings, but search engines do and having a lot of links to your website will increase your search ranking. However, search engines assign values to links so if your links are all on directory websites which are basically just link-dumps, those are going to have a lower score than if they're coming from people's personal blog sites, for example.