MySpace trackers, as mentioned previously, are like MyBlogLog for MySpace pages: users insert code into their pages and their accounts log visits from other users of that service. For non-users, only the location of the visitor is reported. Trakzor is a popular offering.
MySpace isn't always happy with the way these services operate, however, and can shut them down by blocking the embed code. Who's On My Page suffered this problem multiple times, constantly changing the embed codes to avoid filtering, but the issue just kept repeating itself. The solution is a Firefox toolbar, meaning there's no longer any need to embed code into your profile.
Another big change: Who's On My Page now supports Bebo, Friendster and hi5, in addition to MySpace and Xanga (those were already supported). The use of a toolbar allows them to integrate other features, too, like sharing videos from YouTube and other popular sites.