For me, I get beta testers for a site I'm working on through my blog. I never advertised it (other than a link in my "Projects" page), and admit up front that

For me, I get beta testers for a site I'm working on through my blog. I never advertised it (other than a link in my "Projects" page), and admit up front that it isn't even beta-quality yet. Still, I get about 10 signups a month. Write a blog and publish the articles on social networking sites. You'll get beta signups naturally from that.