SEO Spyglass is most effective tool for find out the back links the site has and also helpful for selection of appropriate keyword for optimize the site. It also monitor your competitor's page rank. It allows you to see the alexa traffic rank of every back link.
I have personally tried Spyglass and I agree that it's an efficient tool to check out the number of back links of a website. I also find it a useful tool to obtain appropriate keywords needed to optimize a website. Furthermore, it can monitor your competitor's page rank and updates you the Alexa traffic rank for each of your backlinks.