A friend and former colleague did a presentation a few months ago about “Building a Full-Text Search Engine from Open Source Components”. The slides are up. From the abstract:
In addition to the many useful open source applications that are available ready-to-run, there are quite a few open source APIs out there that are just crying out to be combined in new, useful, and interesting ways. By “just” writing a few lines of code to join them together it should be possible to build a new application that has a unique set of features.