The latest developer release of Mac OS X 10.7 Lion includes a Safari web browser with do-not-track features built in. Do-not-track features in a web browser allow that browser to send messages to websites and advertising networks informing them that the user wishes not to be tracked. However, the feature is only effective if the advertiser or website agrees to not track a specific user, and right now advertisers and web sites are not legally required to honor the wishes of users using do-not-track features in browsers. As to be expected, major online advertising networks are dragging their heels saying they haven't yet agreed on how to implement do-not-track systems effectively.
With the do-not-track feature built into Safari in Lion, Apple joins the ranks of Microsoft's Internet Explorer and Mozilla's Firefox as do-not-track browsers. Google's Chrome is the only major do-not-track holdout. That should come as a shock to no one, considering Google is in the ad business. However, if you do use Chrome, you can download the free do-not-track extension Keep My Opt-Outs.