psSiteSearch is a search engine module for your DNN website, Office and PDF documents. It is a direct replacement for the built-in search engine and gives real results. It is based on Lucene.net and includes a web spider which is used to crawl and index your site.
Search Boost is a highly tuned search solution that radically re-imagines the standard DNN search. It is the best alternative if you want more control over what gets searched, who gets to search, and what results you see.