Yahoo Answers, Quora, niche forums, blog/post commenting, SEO, and so on.
However, these take time as you need to build up some reputation first. You can't just create a new account in forums and start posting things like "Hey guys, get the free eBook about... here!" it will get reported immediately and your account will get banned as quick as BHW mods banning spammers. The same applies to blog/post commenting and if they've enabled comment moderation then you're out of luck. SEO is great, but takes time and money. Yahoo Answers and Quora needs experience and patience to achieve the results you want. I think they've got some algorithm that hides spammers comments if they're posting duplicate comments, gets flagged by its algorithm, and so on.
Other ways include paying FB page admins or forum admins to advertise a banner that leads to your landing page. You can also try posting it in Facebook groups but I won't recommend it if you don't know what you're doing as your account will get flagged and banned immediately.
Blackhat ways include paying spammers to blast your offer to their list. But it isn't cheap and it isn't targeted so your conversion rates will be bad. Also, you may get reported to FTC and other federal agencies for violating can-spam act. Don't do it unless you have your back covered and you're experienced for it. Can't tell you much as it will be against BHW rules.
Unfortunately, all I can say is at this present time of online marketing getting free leads is pretty impossible, if not, hard. And with Google's algorithm update and all of social media platforms' sophiscated anti-spam systems it is pretty hard to get 'free traffic' or free leads. Even if you do manage to get some it is only temporary before you get banned. Even when it comes to SEO you need quality backlinks - again, increasing your cost. In my opinion, the days of no-cost setup are gone. Again, that's from my experience from the past several years. Don't take it to heart.
Maybe you can think ouf-of-the-box and try offline advertising too? I have several ideas but kinda tipsy and lazy to type them now.
Good luck!
