Solr can do this. With it you can define a datasource to crawl it and it interprets the data. Solr is gratis and open source.
Solr has an extremely fast search engine (Lucene) and can import a lot of data. It is optimized for "field like" data like
HTML but there are converters of everything that contains text (like Word, PDF, etc.)
If you just need a few intranet sites (<100.000) to be indexed AND you can access the database you should just set up the
Data Import Handler to do the work for you.
Else you will need to code some own thing that sends the data to the server.
Keep in mind that this is a
development tool and not an end user
program. You will need to create some interface. (It is quite easy, in my experience)
If you store the information in various fields (like
filetype...) you can search through this fields specifically, thous having the possibility to narrow your search down. If for example all your sites in the intranet have an author and you can access that field and index it, you can search for all documents that are from this author, but ignore all that are about him. It also supports
fuzzy search ("seach" finding "search")
I used Solr in one (and the underling search engine Lucene in many) projects and was very impressed by it. The high flexibility in the data processing engines are incredible. The searching part is so fast that I have it on my list to one day understand how it works :)
If all you need is a search crawler & search interface, then the configuration overhead of Solr might not be what you need. But if you need a tool that chews through 30.000.000 documents than this is the tool to go. In the project I used it (with said amount of documents) we had more trouble with network latency then with the Solr search time. You can replicate the index and use a load balancing Solr instance that distributives the search requests to the other ones. And and and... the amount of different optimizations of this tool are staggering. That of course comes with a bit of necessary configuration that might not be super intuitive.
As hinted above,
Solr is a wrapper around
Lucene so if you already have a
CMS to do the site creation for you there might be already a
Lucene plugin for it that you can tap into.