GNU mifluz has two main characteristics : it is very 
simple (one might say stupid :-) and uses 50% of the size of the 
indexed text for the index. It is simple because it provides only 
a few basic functionalities. It does not contain document parsers 
(HTML, PDF etc...). It does not contain a full text query parser. 
It does not provide result display functions or other user friendly 
stuff. It only provides functions to store word occurrences and retrieve
them. The fact that it uses 50% of the size of the indexed text is 
rather atypical. Most well known full text indexing systems only use 
30%. The advantage GNU mifluz has over most full text indexing systems 
is that it is fully dynamic (update, delete, insert), uses only a 
controlled amount of memory while resolving a query, has higher upper
limits and has a simple storage scheme. Consuming more disk space 
allows all this. 

WWW: http://www.gnu.org/software/mifluz/