Although these are viable short term profit mailing list strategies, if you're looking for a longer term sustainable solution, you need to be building a mailing list. A mailing list is a set of people who have opted in and agreed that you can mailing list contact them. Such a list can be highly lucrative. Because they have actively decided to let you market to them (that is, you are not sending spam), you can contact them about products whenever you want.
In other words, unlike a visitor to your site, who might buy, these are long term prospects. Why sell to somebody once, when you can sell to them repeatedly? For many people, the most difficult part of gaining such a list is actually mailing list getting prospects to opt in. The traditional approach is as follows. You create some kind of free offer or product. You offer this on a site that you own.
People who wish to download that product have to opt in to your mailing list. But how do you get people to know about the free gift you mailing list are offering them? The method I highly recommend is known as giveaway marketing, and can be used to grow yourself a giant giveaway list. A giveaway event is where a number of like minded marketers come together to offer their products, all through a single main web site.