This looks pretty cool.

http://blogsearch.google.com

Info on how it is set up is available too. Looks like it indexes RSS/ATOM with Google's engine. Unsure what it does with categories or "tags". But this is interesting:

What search operators are supported?

All of the standard Google Search operators are supported in Blog Search. These include:

• link:
• site:
• intitle:

Additionally, Blog Search supports the following new operators of its own:

• inblogtitle:
• inposttitle:
• inpostauthor:
• blogurl:

For example, a search such as [mandolin inpostauthor:Graham] will show you posts about mandolins written by people named Graham. Note that you can also use the Advanced Search option to achieve the same effect.

The operators to seach in post title or author could come in handy when trying to remember a post that has slipped from the aggregator (which happens often). Or for a post on a particular blog. Try [Paul inblogtitle:ricoblog inposttitle:corinthians] for some fun. Note that indexing goes back to June 2005.