There is a popular technique for recognizing visitors that involves setting a permanent cookie. Although it sounds simple enough, it's not as trivial as installing a traffic analyzer. It requires a little customization to the site. However, the reliability compares well against anything gleaned from logs.

An alternative is to look for unique user-agents (browsers) and IP addresses. Although it's easy enough for a traffic analyzer to do, this idea is hideously unreliable due to the routers used by corporations and the way big ISPs distribute traffic. One of those problems causes many visitors to look like one and the other makes one visitor look like many. There are ways of alleviating this (such as applying a Class-B subnet mask) but it's never enough.

Despite the reliability problem, it hasn't been enough of a problem to deter some of the traffic analyzer companies. Perhaps they experienced a consumer demand that we haven't had the chance to experience yet. But for now, we prefer not to risk recognizing visitors.