Ask Slashdot: Command line interfaces, what is out there?

Mars729 writes: GUIs are walled gardens in that features available in one piece of software is not available to other pieces of software. However, there is software out there with command-line options that can make software features accessible to power users and programmers.

Some important ones I have uncovered are:
Exiftool: A command-line application that can read/write almost any kind of metadata contained in almost any filetype
Imagemagick: This and similar software like GraphicsMagick is a full-feature toolkit for displaying, converting and editing image files.
Irfanview: Like Imagemagick but faster, although it has much fewer features.
FFMpeg: For video files
VLC: For audio and video files
Aspell: A command line spell checker
Google Static Maps API: A URL with coordinates, markers, zoom levels and other options to show a custom map from Google Maps. (I just uncovered this: no need to learn KML!)

Less useful but still useful are command shells. These provide file management mostly. I believe some of them may allow for sending and retrieving email messages.
Also useful but less accessible and with a steeper learning curve are software with APIs and scripting. Examples would be Visual Basic for Applications in office software and groovy scripting for Freeplane.

What else is out there?

I am currently creating a GUI that provides menuing tools, grids for database-like stuff, a non-hierarchical data structure, a set of commands and a media organizer. I am currently trying to get the media organizer working. Slow work, I don't have much free time to work on it. If I ever finish this thing it will to a limited extent allow a power user to create their own customized GUI with a minimum of programming. For myself, I want it to manage my nature observations ... checklists, photos etc. The media organizer (photos plus voice files mostly) is what I am most keen on — the photo organizers out there generally suck.