If you are a blogger or have a website and trying to popularize it, certainly you would like to check for who all are linking back to you on the world wide web. This is how Search Engine decides how popular you are on the web. And if you are just starting, you can try build the back links and be ready to spend few bucks for that initially. You can also take help of viral tagging recently getting popular among webmasters.
If you ask me, at least I always like to check the back links, and I have found a very good page rank tool, Free Back link Checker. It gives details on the Link popularity, IP popularity and Domain popularity. Certainly it has some limitations like maximum of 2000 records or you can get the latest report once in 5 days, but easily ignorable drawbacks. But then I always love to get the report and sort the back links with page ranks, so I would know who are there to help to build my PR 
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Thanks for the tool. I don’t have any backlinks yet, since my site is so new. Hopefully the ones from Wikipedia get noticed by Google soon