Every so often, I'll get a call or email from an attorney asking to explain the metrics he or she found on a "back link" checking website. Most of the time, they found the site by performing a Google search for "back link checker" or "page rank checker". A list of questionable websites appear, most have a space to enter your website address, and most are surrounded by flashing banner and text ads. Here are screenshots for the first couple of results on the search "back link checker":
What These Tools Are Doing
The majority of these websites are either plugging into another tool and delivering its results or they are performing an advanced search through a search engine. Some plugin to the now defunct Yahoo Site Explorer tool (which means if you see a count of Yahoo backlinks this number is no longer accurate). Others simply pull your Google PageRank number, which has its own set of problems as far as being a useful metric.
While the idea behind the stats being presented has merit, if you aren't sure where the data is being pulled from it's difficult to make good assessments and decisions.
My advice is to stay away from these spammy sites and use tools that are reliable.
A Reliable Tool To Use
While there are some excellent paid tools out there, one free tool you can use to get a better understanding of your back links is Open Site Explorer from SEOMoz. This tool essentially crawls the web and tracks link metrics. Open Site Explorer then allows you to enter a url and see the websites and types of links pointing to the site you entered. While the results aren't perfect, it will give you a good indication of your link profile and it is more trustworthy and reliable.