If you have a topic that you are passionate about and you build a web site about it, then this option is one for you. Find a forum that matches the topic of your web site and start helping other people with your knowledge. On most forums there is a possibility to have your own "Signature", where you can have one or more lines of text with a link that people can click on.

For instance, the site http://forums.digitalpoint.com has a lot of requests for information on Joomla! where you can help people to solve their problems. After a number of replies to questions, you can put in your own signature such as:

The links you put in there will not only bring traffic from your posts, but they also count as incoming links for Google. Not all forums have the same rules—for example, some of them have a rule that you can put a link to your site, but it should be the URL only without your main keywords in the link.

So, be sure to go to the best forums you can find on your topic and start helping others with your knowledge. In the meantime, work on your incoming links as well.