Baloo is the file indexing and file search framework for KDE.
Baloo focuses on providing a very small memory footprint along with with extremely fast searching. It internally uses a mixture of sqlite along with Xapian to store the file index.
Contributing to Baloo
The Baloo developers maintain a list of currently active tasks over here. Additionally there is also the bug tracker.
What about Nepomuk?
The Nepomuk project in KDE aimed at implementing the vision of "The Semantic Desktop" using RDF. After years of trying the developers felt that while many of the ideas behind "The Semantic Desktop" were great. Implementing them using RDF and classical semantics was not the way to go.
The Baloo project is just a file indexing solution for KDE. Everything that Nepomuk provided is now handled by other software which are part of the KDE Umbrella.