Back link analysis tools are some of the most interesting to look at. I have built one, that I am not sure if it works anymore, on August 11, 2004. Since then, Google messed up the link command to show a "sampling" of all links to a particular page. So the scraper tools came back.
Anyway, Aaron Wall, from SEO Book, released a new tool last week he named Back Link Analyzer Link Popularity Software (Beta). I gave it a quick try and its pretty cool, its a lot like the OptiLink Software but free. Aaron is asking for feedback for ways to improve the product.
I am sure when Yahoo! Site Explorer comes out, it will lead to ways to improve Aaron's tool.
Forum discussion at Search Engine Watch Forums.