Ready? Here goes:
For this method you'll need to find a large social group. Yahoo groups works great. So does myspace categories, or any large social group or newsgroup.
I went to Yahoo groups and because I'm a guitarist, I browsed through all the guitar groups and found one with over 9000 members, ranging from beginner to expert guitarists. I joined the group and lurked for a few weeks to find out who the experts were in there - they all think they are by the way.
After some time, I posted a thread saying I was going to purchase my first effects pedal (a device guitarists use to create funky effects on stage), and that I would appreciate it if they could give me any tips or advice on helping me decide which effects would be useful, and what type of music I could create with them etc.
Next (this part is important), I told them I would gather the collective opinions of everyone, compile it into a short report that I will give out for free if they gave me their email addresses. I also said that even if you didn't have anything to contribute to the thread, they could still leave their email address or PM me and I would send a copy of the report to them. Within about a week, I had more than 1000 replies and I put together a short report with the replies on the board titled "Must-have effects pedals for the professional guitarist".
I also made a fancier version with a bit of editing - throwing in some information from PLR ebooks on guitars and some pictures. Inside my free ebook was an upsell on the first page and last page to buy this better, full version for $10. This was a price point I knew would appeal to them, but the price was low enough to ward off potential returns.
For mailing, I couldn't use Aweber since there was no double opt-in, so I used Interspire Email Marketer. Interspire is kind of expensive, but IMO the price is well justified considering all the features they give you. That said, there are other free mailers out there as well. Just do a quick google search.
Inside of a month, I had about 300 sales (do the math), which is impressive considering you're not "supposed" to be able to make money off newsgroups. But that's just one quick and easy way to make money off it, but the possibilities are endless. How about sending them to your newly-started forum? Membership site? Training program? They take longer to setup, but they will make you even more money in the long run!
Its virtually impossible to saturate this method, since there are SO MANY niche groups out there.