Drill
Drill is a free and open source software to find files without indexing with smart scanning
FSearch
FSearch – free and open source, fast file search utility focused on performance and advanced features for Linux
Synapse
Synapse – free and open source, simple and fast a semantic launcher for searching and launching applications and files
Recoll
Recoll – free and open source software for full-text search of files by content, as well as by file names