PageRank is a system for ranking web pages. Normally you need Google Toolbar in combination with Internet Explorer to find out the PageRank of a site.
A few days ago, someone mentioned Proogle on the Usenet newsgroup alt.internet.search-engines. Today I had a look at this site, which is somehow able to display the PageRank of each page it finds. The site calls Google internally and also uses some method to obtain the PageRank.
If you use a query like site:yoursite.com you get a quick overview of the PageRank of each page. The Google modifier site: has been changed some time ago, it is no longer needed to add a keyword to get results.
I have no doubt that Proogle is a violation of Google's terms for using PageRank. The site also uses almost the same layout as Google, and replaces the original advertisements. So it might be gone in the near future.
Update May 21, 2004: Proogle has been renamed to Prog and has a new address.