I use Google tool bar only its really very help to me... in this tool we can able to find easily what is our site PR (Page Rank)
Although toolbar PR is worthless to me, I still check my web page's PR, because when selling text links, my buyers always asks for the PR of the front page. The tool that I use is prchecker.info. It's the first result that I found on Google when searching, so I've always used it to check my page's Page Rank,
Dude, I use Google Page Rank Toolbar. I think it's a reliable tool and ever give you the current rank of your site.
You should use Google's toolbar it self, which is the original source. How ever you can also use Firefox's plug-in named Swoosty to get both Google PR and Alexa's Ranking.
My tool library has many different tools some are....Link checker for checking links,GoStats for web analytics,Google Adword keyword tool for keyword.