What if, you found a great web page that lists a lot of interesting links about topic of your interest, but you don't have enough time to read everything. A good option would be to create a custom search engine, but it's too time-consuming. Another, way is to create a custom search engine on-the-fly from all the links included in the page you've found. That means you'll have a search engine restricted to all the domains, subdomains and web pages linked from your page.
For example, del.icio.us search results pages are a good place to obtain a list of authoritative sites on a topic like AJAX. Once you have the del.icio.us URL, paste it at Google's on-the-fly search engine factory and enter your query.
Other good starting points could be: Wikipedia articles, other wikis, directories, scholar papers, pages with many references etc. Maybe Google should create a separate search engine for pages that have a lot of external links related to a topic (like these pages).