Sphere is a new blog search engine that has many interesting features. You can find the most relevant blogs for a search, not just the most relevant post using "Featured Blogs". And because the featured blogs are determined algorithmically you won't find sponsored listings.
You can search blog posts written in a custom period of time or even in the last hour. For every blog, there is a profile that lists some statistics: average posts/week, average words/post, average links/post, number of posts in the last week, number of backlinks.
Sphere has a nice bookmarklet you can drag to your browser's toolbar: Sphere it and that helps you find blog posts that match the subject of the article you were reading.
Unfortunately, the relevance of standard search isn't that good. In a search for [internet explorer 7], the third result was the first relevant to the search, the first two being a collection of links to other posts. Sphere ranks high archives and pages that contain all the post with a tag, but this will probably be fixed. Another problem is that only the last four months of posts are searchable.
"Sphere, a San Francisco- search engine company that specializes in searching blogs, for relevant information has closed a $3.75 million in fresh capital from Hearst Publishing, Trident Capital and About.com founder Scott Kurnit." says GigaOM. So Sphere has the money and the potential to become a great blog search engine.