I've just been playing around with Google Co-op's "Custom Search Engine" feature. I'm interested to see if it's possible to build a decent topic-specific search engine given the controls exposed in that product (mainly, url whitelisting).
The test topic area I'm starting with is, not surprisingly, software engineering. I've added a half dozen sites, mostly blogs, related to software and software engineering. You can try it out right here, using the box there on the right.
One of the more interesting mechanisms is the ability to invite collaborators to contribute sites. If you read this blog and you have sites you recommend for software-related information, drop me a line or post a comment, and I will add you to the collaborators.
(Program note: while adding the custom search box, I also re-enabled AdSense on this blog, because it seems to have disappeared.)