Go to: https://siteexplorer.search.yahoo.com/new/
Enter the URL that you want to now more about and click "Explore"
Click the "InLinks" button
Change the drop down box which reads "From All Pages" to "Except This Domain." This will allow you to see all of the external links that link the domain. Click, "Explore."
Now...poke around! You will see a list of websites that link to your competitors website. Are they from high quality websites? Are they from spammers? Are they from influential bloggers? You may want to get links from some of these websites as well...or not.
You may want to read deeper into the links to see what people are saying about the website.
I can tell you that I have found some very interesting things using this tool. I have found fraudulent Wikipedia articles and caught a competitor who was using the services of a well-known spammer. I was also able to identify a highly-placed blogger in a certain niche and cultivate a relatonship with him.
And, of course, you will want to explore your own website links and find areas that you can improve. There are many other uses for Yahoo's free site explorer, I just wanted to give you one important use. And let me let you in on a little secret that most SEO's already know: Use Yahoo to check your links, not Googles. Google's link checking tools are limited and do not give you a full-range of results.