Yes Google Analytics have this feature to knew where the traffic comes for a site. It is a very Great free tool provided by Google. You, can analyze now that the traffic came for your site by this thread.
I second, third and so on the people who say Analytics. I have implemented it in my sites and it helps keeping track of my visits. Not to mention that, when I try to sell a site or just advertising, any potential client wants to see my analytics stats. It's some sort of a "norm" now, so I recommend it too. Not to mention it's easy to use and FREE.