Google sitemaps gives a rough feel for your pageranks in your site. In principle, this data can be used to estimate the actual PR of the site. You can help calibrate this relation by filling in this form. Thanks!!

Under Google Sitemaps → Statistics → Crawl stats you can find the "PageRank distribution of your pages in Google":

Sitemaps will also tell you which page has the highest PageRank (it isn't necessarily your home page): If you look at the source code for your sitemaps/statistics/crawl page, you can actually read off the numerical value of the fraction of pages with different page ranks. If you do, you'll see something like:

This will tell you, for example, that 21% of the site's pages have a Medium PR.

So, if you have the data ready, you can proceed to fill in the form, and help calibrate Google Sitemaps pagerank info! Once I collect enough submissions, I will write the tool (which will of course be free for all the community to use). This tool could prove to be more accurate than other tools available. It's advantage over the toolbar PR is that the latter updates every 3 months or so, while sitemaps PRs update much more often.

Thank you for your cooperation!