Well search engines like google, etc are probably the most cost effective way of advertising your site. Networking with other similar sites and exchanging links.
I tend to stay away from anything involving popups and popunders as I find them annoying.
When you post at forums, use your URL as your signature link (providing it is not an adult site), that can attract visitors.
Also depending on the nature of the site, you may want to look at banner exchange, and top 100 sites.