An very intriging new search for the Stadsbiblioteket in Denmark is called SUMMA.
They call it integrated search instead of federated search and plan to become Open Source in 2008. It was amazingly fast and sophisticated.
Come to think of it, the story reminds me very much of our own Livetrix development at Groningen. It was a good presentation and rumours were whispering that it will be used in a few university libraries in the Netherlands.
Summa is based on user studies. One such study is The hybrid library: from the user's perspective. [pdf 22 mb)