One of the best search and advanced engines for Joomla! 1.5 is already fully free – JXtended Finder (jxtended.com).
It can index and search a variety of content, from Joomla! core articles and contacts to JXtended Magazine articles, authors, issues, publications and more. It has plugin system which allows administrators to easily install new search adapters which provide support for virtually any type of content, including standard Joomla! content and other third-party extensions such as: DOCman, EventList, JomSocial, K2, Kunena, VirtueMart and ZOO.
Finder pre-searches your content so that it’s fast and accurate. Search results are scored dynamically based on the context and frequency of search terms. The most relevant content will always be listed at the top of the results.
If extensions can’t find any results, it will look for other words in your content that sound like (based on phonetic English spelling, only) the user’s search query and suggest them as alternative queries.
Finder can be download with all plugins from: http://jxtended.com/downloads/finder.html