Bit of a blog search shooting match going on between old standby Technorati and new kid on the block, Sphere.
Technorati looks at link structure while Sphere casts the net a bit wider looking at the text on the page to get more contextually matched pages that may not be linked via an explicit url.
I’ve have both Sphere It! and Technorati This! (sheeesh, that was hard to find that link) bookmarklets side-by-side and find the two actually compliment each other. I use "S" when I am looking for conversation around a very broad topic and am looking for the fuzzy cloud of buzz around it and I use "T" when it’s a specific topic (or flash/video site with little or no textual info such as the new nikeplus.com site) and I want to hone in on just the people linking-to-that-very-URL.
Two things I like about Sphere are the narrow time windows (last hour!) and the cool flash-based measuremap slider thingy that comes up when you select custom date range. By the way, this widget is available under a Creative Commons license from the kind folks over at Adaptive Path who worked on both Measure Map and Sphere.