Backlink Checker is a 100% free desktop tool the allows website owners or SEO's to check a websites backlink profile. It is powered through the MOZ API (which is free to use). You simply enter the web properties (domains / webpages) you want to see the backlinks for and the tool will fire off requests to the MOZ API fetching all the known websites / webpages linking to those web properties.
Links are still the core of Google's off page ranking algorithm. After on page SEO, links should be the next variable you focus on. Seeing who is linking to you and who is linking to your competitors can really give you the edge over your competition. If you can see where your competitors are obtaining their links you can replicate this and outrank them.
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I highly recommend trying out their free trial. Here are just some of the awesome capabilities of this service.