Stumbleupon, Google and referral links from other sites. I make sure that my site contains relevant content and keyword rich articles. I frequently check my analytics and get ideas from there as to which topics generate high traffic. I write more articles on that particular subject. So far, this strategy works wonders for me.
The main source of traffic on all of my sites is google. After google its Yahoo, Youtube, stumble and a little msn. A little traffic comes from signature links, but not much.
google twitter facebook social networking bookmaking the article directory press release Comment on blog
post comments on the blog is my biggest source of traffic it allow me to generate targeted traffic to my blog