- 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?"