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.
Baloo is a daemon to index files. Applications can use the Baloo framework to provide file search results. Used by KDE desktop applications.
Core of GNOME desktop. Has built in search feature, with a public D-Bus search provider API that allows any desktop software to extend the Shell's search functionality.
There is a Tracker search provider extension. Some applications already expose data from Tracker as search results: GNOME Documents and GNOME Photos.
Serd and Sord
Serd is a lightweight C library for RDF syntax which supports reading and writing Turtle and NTriples; Sord is a lightweight C library for storing RDF data in memory.