I checked out two very different site comparison tools yesterday, both of which could be useful for different SEO purposes.
The first is Quantcast, which is similar in scope to Alexa. If you look at myspace as an example, you'll see lots of demographic data, numbers and types of visitors, other sites the audience visits, and lots more interesting details. You can compare up to 5 sites. (Hat tip to Chris Rizzo).
The second is Delexa. This site uses the top 50,000 sites (as determined by Alexa) and uses del.icio.us tags to rank them. I'm sure there are various uses for this data, but the one that stands out for me is that it can be used to find high quality links in the topic area of your choice. For example, let's say you were looking to obtain backlinks in the area of seo. Searching for [seo] as the tag, you will get this result. You now have a nice list of sites that are both topically related and likely to be high-powered. Obtain backlinks from some of these sites, and you might have some sweet results.