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.
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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).