There are some open source alternatives we're exploring for site indexing. We want to build an internal webservice like freefind.com to offer search on our network of sites.
Here are a couple of packages we're looking at and some other relevant information.
http://www.aspseek.org/
http://swish-e.org/
http://www.searchtools.com/tools/tools-opensource.html
http://www.searchtools.com/
You can compare a few of the above in action at:
http://search.ala.org