Alexa, the free traffic statistics site, has just upgraded its service. Alexa has been gathering data from users of its toolbar for many years. Yet the cumulative statistics of traffic to web sites was not available for more than two year periods.
In adition to improved features, the new upgrade offers a max button that offers its entire statistics history of a site.

One of the most popular alexa statistics feature is the ability to compare traffic from two sites. For instance comparing ebay with amazon traffic results in the following graph

On lower traffic sites the statistics can be distorted, and errors can be quite large depending on the alexa's base of toolbar users. Other than that, Alexa is an excellent alternative to payed traffic statistics services like Netvalue from ACNielsen or Hitwise.