Google launched an interesting way to search for custom search engines created using Google Co-op: a Google Base structured search that only includes the most popular search engines. The cool thing is that you can restrict the results based on the search engine's name, language, its description, the most popular queries or the number of sites that are included. "If you're interested in finding a search engine to contribute to, search specifically for search engines that allow volunteers. For instance, if you're most interested in non-profit organizations, search only for non-profit search engines," suggests the CSE Blog.
Google Base has a very powerful search interface and it would be nice to see it in other products like Blog search, Gmail or Google Calendar. When you perform a search or select an option, Google Base will only show the options that are available in the current state and that's useful to dynamically build a complex query.