apt-xapian-index is a Xapian-based index of package information.
Current uses
apt-xapian-index, a package providing the update-apt-xapian-index tool to keep the index up to date in a Debian system
aptitude from experimental can use apt-xapian-index for very fast queries
Resources
The first of a series of blog posts about apt-xapian-index, including many commented code examples showing various ways of accessing the index.