Obsessing about fluctuations in Google PageRank may be a complete waste of time but still, the PageRank values of individual web pages on a site can give you some good idea about the relative importance of these pages in the eyes of Google.
For instance, if there are two or more pages on your site with very similar content, find the one with highest PR and then do a 301 redirect on the other pages. This may be a slightly vague example but you should get the idea.
Brandon Thomson has created a PageRank Checker service powered by the Google App Engine that lets you calculate the Google PageRank of multiple URLs in one go. You can either upload the XML sitemap file of your website or create a simple text file with a list of all page URLs and upload that to PageRank Checker.
The service can also help you find the Google PageRank of all URLs that are linked from a particular web page.
Related: Find PageRank on Google Data Centers