Link popularity is a highly important part of most search engine algorithms for determining your website's ranking. Link popularity refers to the quantity and quality of inbound links pointing to your website. Search engines assess the quality of the links by checking that the website that is linking to your website is of a similar subject matter (so if your website is about potatoes then a link from a website about motorbikes isn't as good as one from a website about carrots - a bit simplistic but you get what I mean) and the inbound link text used to link to your website (so, if you are optimising for 'potatoes' and the inbound link text is 'carrots' then this isn't as good as one that says 'potatoes').
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Link Popularity Checker checks your link popularity across a few search engines and outputs a pretty impressive report. It even lets you check against up to 3 competitors.