A "search engine" ahum built from the ground up.
The goal isn't to make the fastest, most robust and reliable search engine ever (everybody knows AltaVista did that already) as much as to write a simple and elegant search engine.
I already implemented a search engine for binary queries (with postings list in linked-lists) in Java, and those search engines are not common nowadays so I think I'll focus on implementing a decent full-text search engine using TF-IDF to describe each documents and query.