This is a metadata indexing and searching system similar to the various Spotlight, Beagle, etc. All the names actually used (for example for the MDFinder application) must be considered provvisional; I’d need a single name for all the system and some good icons...

Extracting metadata 

To be searcheable, the metadata of your files must be first extracted and indexed. 
Launch SystemPreferences and click the Indexing icon. 
The indexable paths view shows the directories that can be indexed; the default values are your home directory and the GNUstep Documentation and Headers directories.  In the excluded paths view you can add subdirectories of the indexable directories that you don’t want to be indexed.  
Also all the file with a path extension listed in the excluded suffixes will be excluded by the indexing process.
To begin indexing select the enable indexing  switch and press the Apply button.
This will start the mdextractor daemon, that is, the process that will first extract and then keep updated into the database the metadata contents of your files.  Clicking the Show status button will show a window where you can see the status of the indexing process.
To extract text contents and other metadata attributes mdextractor uses a set of extractor bundles that actually understand these kinds of files: Abiword, Application (GNUstep application), Html, Jpeg, OpenOffice (writer),  Pdf, Rtf and Xml. A generic "Text" extractor is provided for the other files (if they are text files).

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The "Paths" tab of the SystemPreferences "Indexing" module.

The first time you run it, mdextractor will take some time to extract and index the contents of your files; this time can vary from a few minutes to some hours depending on the number and the size of the files; for example: if you limit the indexable paths to the GNUstep Applications, Headers and Documentation, the indexing time will be only one or two minutes; on the other hand, indexing about 2GB of sources - as I’ve done for testing - needs about two hours.
The database is available for searching from the beginning of the indexing process.
When the indexing is done, mdextractor will continue to update the database in background.

Searching

To perform a new search you must start the MDFinder application and choose New from the File menu.

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A MDFinder search window.

A search can be saved to a file and loaded again into the MDFinder application.
All the open windows will be updated automatically when the contents of the database change.

Choosing "Other..." from any of the attribute popup menues displays the Attribute Chooser window where you can view a description and select an attribute that is not present in the popup.

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The Attribute Chooser window.

In the Search Results tab of the SystemPreferences Indexing module you can choose the order used to show the results.

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The "Search Results" tab of the SystemPreferences "Indexing" module.