The over-riding question is why you do you want backlink data? Let's assume the reason is because you want better performance in organic search - probably Google. The question then becomes, how do I find backlinks that increase performance in Google?

We then need to define the characteristics of such a link:

-          It will pass anchor text benefit

-          It will pass PageRank (for content discovery, and also to increase the weight of anchor text)

Note that links can get your page spidered and indexed quicker (content discovery), but not necessarily pass anchor text (and increase rankings).

Anchor text benefit is somewhat easier to measure than PR (PageRank) - since the actual PR value of a page is only known to Google. Toolbar PR is not the same as this value, and is a highly unreliable method to try to gauge actual PageRank. Both are easier to measure when looking at specific links, but are very difficult to measure in aggregate.

Onto the backlink data

The only search engine that provides publicly accessible link data of any use is Yahoo. Similar data is also obtainable via a verified Google Webmaster Tools account for the site. The overwhelming majority of free tools online that look at backlinks in any detail are either scraping (download pages from a site using a script or robot) data from Yahoo, or accessing data via the Yahoo API. However, there's a significant problem with the data provided by Yahoo and Google - there's no indication whatsoever that the links they return are actually passing value -  for either PageRank or anchor text. Indeed, both will return links with the nofollow attribute and from sites that can be verified as unable to pass value. Yahoo also includes hotlinked images, links to javascript and other media files which are highly unlikely to be a good target for link building.

Theoretically, you could build a tool that analyses the data and tries to estimate value and aggregate anchor text - we've done similar ourselves in the past and it's not a worthless undertaking, but it's still some distance from having actionable data, as defining and measuring those criteria requires a bigger picture than backlinks for a single website. There are also problems associated with starting an analysis based on questionable data - it requires a lot of processing that should be unnecessary, if nothing else.

Enter third party backlink databases

There are two major players in the backlink database world - Majestic SEO and SEOMoz's Linkscape. Both are subscription-based tools that have a large database of links, and so can analyse them as a whole to try to work out value. Our own Dixon Jones wrote a side-by-side comparison of the Linkscape and Majestic SEO early this month. For simplicity's sake, let's just say that Linkscape has a better interface and (in my opinion) much better measures of quality, but more limited data available. Majestic has a much bigger database, and lets you export the whole lot to examine and analyse yourself.

Frankly, no link tool is going to spit out the "best" links to acquire, partly because it's still a numbers game as far as most tool are concerned - usually the total number of links. The better number to use - the value of a link - is at best unreliable. A tool might highlight a thousand links with the right anchor text, with apparently high value, but Google might say otherwise. In many cases, a single link that is "right" is going to outweigh the actual impact of a thousand links that are wrong.

All backlinks are not created equal

A further problem with tools is that they can only ever play "catch up" to the type of processing Google uses - the position of a link on a page, the context it's used in, and the traffic following links are also aspects Google has data on and can analyse - and I've yet to see a link tool attempt to gauge any of those things. There are many considerations well beyond the scope of most automated tools.

Tools collect backlink data, people analyse

Receptional have always advocated a "human" approach to online marketing. Of course, we use free, commercial and in-house tools, but we always see those primarily as an aid to data collection, as opposed to the core of an analysis of data. Tools like Majestic are an unprecedented way to acquire large-scale data about the interlinking between sites on the web, but it takes an experienced link builder to take that data, and turn it into actionable information. Better still - that expertise can directly translate into a link building strategy - from filtering undesirable data, to spotting patterns that point at the right audiences to target. A good link builder will know intuitively which parts of the wealth of available data to focus on.

The best tool to find backlinks is...

...an experienced link builder. Let them handle the tools, the data and the analysis - and translate that into a way to use backlinks to help your site reach its goals. Of course, this is something you can learn yourself. Be prepared for a lot of reading, a lot of data (and spreadsheets!) and a lot more testing. Start by trying all of the tools available, and aiming to see if you can find evidence that their measures of value are reflected in actual performance. And of course, if you decide to hire someone to handle link development, remember that you're paying for expertise, not tools - look for someone who understands the complexities of link acquisition, and will be able to advise on the right link strategies to pursue to reach your goal - be that organic search performance, additional traffic or branding considerations.