MG4J is a project that aims to provide facilities to index and query vast quantities of information. It is the kind of thing that will allow you to build your own version of GoogleDesktop. To be precise it implements a compressed inverted-index.
The critical feature of the release is its fundamental scalabiliy - both for its simple performance and its scope for multi-index, multi-process handling. All very useful features for a LGPL licensed library.
It is certainly on my 'to evaluate thoroughly' list.