There are many Backlink Checker Tools around. However, most of these backlink checker tools are based on yahoo site explore. Simply go to : http://siteexplorer.search.yahoo.com/ and put the URL you want to check backlink of. Tools like SEO Spyglass, SEO Elite etc use data of yahoo site explore to check backlinks. All of the so called SEO Experts are using these tools.
I used to use yahoo explore to check backlinks about a year ago. Now a days i never visit yahoo site explore. The reason behind is there are tools around which will give us very detailed data of backlinks such as Pagerank of the page, domain age etc. SEO Elite even shows if the backlink is nofollow. The only defect with SEO Elite is it works slow than other tools and you will need very high speed internet to operate faster. I would recommend SEO Spyglass as one of the best backlink checker tools available in the internet. See the image below to have basic idea about the software.
This backlink Checker Tools is absolutely free to use. Trail version comes with the power to check backlinks of upto 1000 backlinks. The harvested backlinks can be arranged by various factors such PR, Alexa ranking, Domain age etc which actually means a lot. Just Click Here : http://www.link-assistant.com/seo-spyglass/ to download the tool for free. If you want paid version it costs only $97 which will allow you to analyze more than 1000 backlinks, save the backlinks and project and many other features. I am currently using the free version and is enough to check backlinks of competitors and my own websites.
Note: The free version of SEO Spyglass only harvests backlinks from yahoo.com. The paid version harvests from different sources.
There is another awesome backlink checker tool provided freely by SEOMoz. Simply Click here : Backlink Checker
Open Site Explore is awesome tool. It can analyze upto 1000 backlinks with the free version and you will have to become pro member to analyze more backlinks and get access to many other awesome SEO Tools.
So what are you waiting for ? These all tools are available totally free of costs and worth than 1000s of $$. You can dominate your competitors using these tools. You can know how many backlinks from which sources are linking to your competitors website. As we all know, backlinks are the main factors that will affect ranking after website content and onpage optimization. It is always best to get natural backlinks by posting very useful content in your website, but it is not always possible.
Use these backlink checker tools now and get some good ranking in search engines. All the best folks.