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Install "Search Status" addon in your Firefox browser. It's the best, free and most reliable tool.

http://pagerank.my-addr.com/ (Fastest, but limited to 100 urls per IP)

http://www.seo41.com/pagerank-checker.php (Unlimited, but sometimes show wrong "fake" PR)

Check out SEOmoz Toolbar for Firefox or Chrome. If you are serious about your SEO, then you definitely want to know about domain and page authority. Those numbers are a better indicator than page rank. A must have in my book.

I like the WebRank Toolbar add-on for Firefox

I tried many tools but stuck with Rankerizer, it's a software that can check SERPs and PR of each page. This if you need to check your sites. For other sites I use Chrome plugin SEO Site Tool.

Oh, and if you need to do that on a massive scale - use new tool Netpeak. This is my friends' new software and it does soooo much you will be surprised. It's free too!!

Hope that helped.