Egothor is a Java based search engine. I've never tried it, but I presume it
is similar to Lucene, which I like very much.

At work we have a Java based Wiki (which I wrote), with the backend using textfiles and Lucene. Using a search
engine instead of a database is an interesting approach to use, and I chose it because I wanted to learn about Lucene (I
don't normally make architectural decisions based on what I want to learn, but I started this on my own time, then showed
it to some people at work, who started submiting patches to tie it into various things, and it just grew…).
I wouldn't recommend using a search engine for financial or other typically relational type data,
but for some classes of applications it works pretty well.

It also allowed me to integrate a lot of the data we have in various locations into one knowledge-base. By using
Apache POI, I index all the MS Word documents available on our
network (I can't remember how many, but the index is 40 meg), and make them available via the Wiki for searching.

The next step is better classification and linking of information. Classifier4J
might help me with the classification, and I can extract some extra linkage data out of assorted task, time and bug databases
we have at work, but I also need to figure out how to customize the Lucene scoring algorithm.