Apps using Tracker
GNOME Books (uses Tracker to find ebooks)
GNOME Boxes (uses Tracker to find VM images)
GNOME Documents (uses Tracker to find documents)
GNOME Files (uses Tracker for full-text search within files)
GNOME Games (uses Tracker to find games)
GNOME Music (uses Tracker to find music and store playlist data)
GNOME Photos (uses Tracker to find photos and store album data)
GNOME Videos (uses Tracker to find video content)
Systems using Tracker
Nearly all GNOME-based Linux distribution use Tracker.
Projects related with Tracker
An RDF quad store that supports SPARQL queries, written in C. More flexible than Tracker's store due to being a "true" quad store. Performance hasn't been compared. Little ongoing development.
Apache Lucene + Solr
A middleware tool that provides indexing and search features. Compared to Tracker, it is designed for much larger datasets, and does not focus on being lightweight at all. It is written in Java and provides a HTTP API.
Baloo is a daemon to index files. Applications can use the Baloo framework to provide file search results. Used by KDE desktop applications.
Recoll is a QT-based graphical application that allows indexing and searching files using the Xapian search tool.
Serd and Sord
Xapian is a C++ middleware component that provides indexing and search services similar to Tracker. Compared to Tracker, it focuses more on scaleability and less on being lightweight.