The easiest way to put your advertising in the Internet on the mass resources (e.g., LiveJournal, Vkontakte, Odnoklassniki, etc.). To publish to add enough just to have an account in the system (registration is free). If your ad has a certain attitude towards any subject, it is possible to find groups/communities dedicated to this topic. To find an echo in communities/groups is easier than just post a link to your page.
Using any search engine to find free message boards focused on a certain subject. After registration you can post freely in almost any ads (unless they violate site rules).
However, with the help of search engines you can find online communities and forums, where mostly you can place free ads (but then again, if it fits thematically).
On large sites where there is advertising, with the administration to negotiate hard enough. You need to have your products (or anything else) fit to the website content completely. Only under this condition, there is some possibility that your ad will be posted on the website for free. Usually to find the contacts you need to click on a special section (for example: communication, contacts, assistance, etc.). Then you need to negotiate. It is likely that the administration will agree to your terms and will publish ads for free.