One critical element of planning for a link building campaign or to do some competitive analysis is to find out how many incoming links you have to your (or a competitor’s) website and what those links are. There are a ton of great resources out there that help you do this. Here are three free ones that I have found to be very useful.
A favorite among search engine marketing experts, Yahoo Site Explorer is great for exploring incoming links to your site, but unfortunately you cannot use it to analyze competitor’s sites because you must validate that you own the site you are scanning by uploading a file or adding a meta tag to your site. Some features include finding incoming links to your entire domain as well as a specific page or excluding incoming links from your own domain. You can also export results to a spreadsheet file if you want to archive or just geek out over your data.
I recently ran across Eric Miraglia’s Inlink Analyzer and immediately fell in love with it for it’s ease of use and because it shows you not only who is linking to a site, but how many links they have pointing to them (A good indicator of how reputable that site is and how valuable the link), and how many times they have been bookmarked on Delicious.com (A social networking site where people can share website bookmarks) and with what tags they are associated with. This can be done on any site, without registering, so it’s good for competitive analysis.
Google Webmaster Tools is a must to set up for anyone with a website. Not only does it have a report for incoming links, but it also allows you to submit XML sitemaps to Google, find top search queries for your site, analyze Googlebot crawler data, find broken links, and many other useful things. I love how you can even see the anchor text (The text that a link says when it links to you, like "Check out this cool site!") of incoming links to your site, but a bummer is that you cannot tell it to NOT show you links originating from your own domain. It also allows export of data to a spreadsheet file.
As I mentioned, there are tons of tools out there that have these features, and many are notable, such as the link analyzer from SEObook.com, Linkscape from SEOMoz, and SEO Spyglass, but they are all commercial software and not free, or they offer minimal information until you upgrade to the paid versions.
You can accomplish the same thing with any of these tools, but the important thing is to make sure you keep building links to your site, and being smart about it.