I had to ask myself this question some time ago. I decided to build it myself instead. Starting point being the principle that Simple Analytics explained in their blogpost some time ago: if referrer is from the same site, then it's just a visit, otherwise it's a unique visitor. I mixed in some more browser details, also used some blacklist of known bots, and now it's functioning pretty accurately. I don't have the tracking that GA does, but that was exactly the point.