Update (2nd May 2006): Sphere has finally launched publicly today. Test it out at Sphere.com, or read Mike's review.
After riding the hype curve for many months, it's great to see that Sphere will soon be alive and kicking (launches in a few hours, I'm told). The blog search engine discovers content based not just on recency, but on relevance, too. This could give it a major advantage over current plays like Technorati. From the about page:
Sphere is a blog search engine focused on discovering high quality, relevant blogs on a timely basis...It starts with relevant results and fast performance. Our new relevance-based algorithm discovers new blog posts as they're created, indexes them within minutes of being published, applies rich semantic analysis and makes them searchable by relevance or time. Plus, we've got a few fun, helpful features that we think make for a richer user experience.
I think Sphere could do well. Also look out for the attractive, uncluttered interface, courtesy of Adaptive Path.