In this question, I talked about main traffic sources for one of my websites.
As you can see, that website receive mainly traffic from organic search (Google, Bing, Yahoo, Govome, Delta-Search, Ask, Babylon, etc.). It's "free traffic". But in my opinion, it isn't so "free", because we have so much work trying to optimize it, creating web presence (a good term for "linkbuilding" ), etc.
Edit: I forgot to say: other of my websites receives 500 visits monthly from Yahoo Answers, so I'm trying to get more YA traffic for that website too.