Google Co-op lets you customize your own search engine which will crawl websites which are based on a single theme.
If you are looking for information on eg. politics or astronomy or weather, you can build a search engine that crawls only relevant websites related to these topics. A great way to reduce clutter during searching and get more accurate results. I experimented with a technology/internet/gadgets-gizmos/web 2.0 search engine which I have named 'DotpiLe', crawling 70+ most relevant websites on the above topics.
I have deliberately omitted del.icio.us because it lists a lot of bookmarks in the results, leading to duplication. It's an open end search engine though, and anybody can contribute.
You can try this tech search engine firsthand here
http://www.google.com/coop/cse?cx=006978851294483185095%3Aphrttmmskjk
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