SEO Tips - How to Check Your Google Page Rank
1. You can download the free Google Toolbar from their website, www.google.com
2. Use a 3rd party tool like, http://www.prchecker.info/check_page_rank.php
UPDATE (9/2/11): If you upgrade to Firefox 5.0 or beyond (presently 7.0 beta is out), you will no longer be able to use Google's toolbar to check Google Page Rank, as they have discontinued the service for newer browsers.
Other alternatives like SEO MOZ exist and are still compatible with newer browsers. Yahoo! has their own version as well and should still be available.