I don't know why, but MySpace, to me, seems like a great form of free advertising, and not many people have tapped into yet.
Simply create a new myspace account, and change the layout to reflect your site, and even throw some banners up on it. Make a pic with your site's url in it and upload as your profile pic.
Once you have your myspace layout setup, then go ahead and do some searches in the myspace groups in relation to your website's content. Join those groups, and maybe make a post informing everyone in that group of your website. Or, merely by contributing to their group, members will see your profile pic with your website's url or site name in it.
Also, there now seem to be blog groups you can join in on too. So make put your site's updates in a blog to inform people that your site has changes to them.
Finally, maybe get some of the myspace profiles of your current members and simply write something in their myspace. A lot of people people browse profiles, and will eventually come across yours.
My Myspace for my forum has been up for only about 2 weeks, I've only requested to be friends with 2 people, and I have 64 "friends" that I have added, that have come across the myspace profile. Also, the total profile views are: 1343.
Attached is an example layout I'm using on myspace (url and sitename removed)
