Google Co-op allows you to create your own search engine. You specify a list of sites and are given a home-page which lets you use Google to search just that list or the whole web but with an emphasis on those sites.
This is really useful for specialists or groups of people who have a set of quality sources on a given topic. e.g. Ruby on Rails developers or horticulturalist. Google Co-op lets just one person or a group of invited people to contribute to the list.
I think it is even more useful as a personal search engine. Here I have created a search list that searches just my resources on the web. It includes my Flickr photos, my del.icio.us and Blinklist links and my blog. Other people can use it but it is more useful to me as a way to pull out past things I have written about, photographed or linked to.
Rollyo does much this but I found their interface a bit complicated and it wasn’t using the great Google search engine.
