Technorati
Want to know what people are saying about your weblog? For $10/year, Technorati will scan a big list of weblogs whenever they're updated and will email you about new links to your weblog -- or provide you with an RSS feed you can use in Tinderbox or another aggregator.
Interestingly, Technorati, Blogdex, and Daypop each tend to find different things.