Technorati serves as a directory of blogs, but also organizes topics through tagging and ranks blogs, according to its own algorithm. It is useful for searching for blogs by category and for finding blog posts on certain topics.
You can browse by category at the top of the page. Just put your over the main category to see the subcategories.
When you click on a category, you can see who the top movers (ie. blogs gaining in popularity) are:
And you can search for topics or blogs, by clicking the appropriate button (“blogs” or “posts”) the search bar:
You can also look at popular tags by clicking “Tags” on the top menu. That will bring you to a page with a huge listing of popular tags over the previous month: