Topical AND Keyword Based Search Engines?

jhughes asks: " I work for a public university and have just recently 'acquired' the project of switching a community Web page (for the city and surrounding counties/cities) to a new machine running Apache. I'm relatively new to Apache but thus far this has not been a problem. The real problem comes from the request of a search engine for the Web sites information. This site will contain a large amount of public information, such as rental property, council meeting minutes, and so on. They are requesting that the search engine be able to scan the entire site for all the data, or specific subtopics (such as 'City Government', 'City Council', or 'City Council Meeting Minutes', with searches limited to each of those areas if need be). To me this sounds similar to what Yahoo has done in some cases (You can search Yahoo, or Yahoo US States, or Yahoo US States California, or specific cities, etc). I honestly have no clue what search engine to use for this. Are there commercial search engines that one could purchase and adapt, or is there a free/open source one out there that would work just as well? There will eventually be a huge amount of information (as all government sites seem to accumulate) online, so it'd need to be able to handle large indexes. Any help, advice, or suggestions would be greatly appreciated."