It is free and easy to do. There is no charge, they accept anything reasonable, and their response is fast compared to certain unnamed directories. This is a tremendous advantage especially on pages with many site listings. The following are excellent resources to learn about the best ways to submit your site to search engines and optimize your site and page design to improve your site's ranking.
Search engines are critical to the success of any public web site.
There are networks that handle selling and serving ads for a commission but most have not been accepting new sites recently. See Adbility for more info. The problem with this approach is that the web site is giving CPC advertisers exposure for nothing. Advertisers signing up for CPC campaigns therefore tend to be more interested in exposure and design their banners to discourage clicking thus avoiding payment. Essentially real-time reporting of impressions, clicks, leads, and sales allow the site owner to quickly see what works and what doesn't.