Where do Pagerank Tools get their Information?

[internetheaven]

So many tools that tell you what your pagerank is if you don't have the toolbar. Why doesn't Google have that function itself? Where do these tool people get their information from then? Is it reliable?

[rainborick]

They all get them by mimicking the Toolbar and querying Google's database. The only difference is that some of the online tools allow you to poll more than the single data center that a user would access via the Toolbar.