Thom has a note about some really nice search systems on his blog. Check there for the details of how these work. The search systems are based on an index of phrases, and suggest results as you type. These, combined with speed of response, give a very good serach experience.
Look here for a 'quick' search of the Phoenix Public Library collection. The dynamic presentation of results as you type works very well. Results are ranked by holdings.
Look here for a quick search of the Library of Congress Subject Headings. This is useful as a way of quickly finding an appropriate heading. Again, results are ranked by the number of holdings in Worldcat. Potentially a productivity enhancement ...
These are pretty special.