I’ve done lots of “useless” software.

  * Lexers, Parsers, Compilers
  * Shortest path, traveling salesman
  * Physics sims, numerical methods 
  * Discrete event simulation 
  * Document formatter
  * Tried to add a GUI Tab pane context and lifecycle to Java CDI. Yea, that just doesn’t work. Lipstick on a pig in a square hole. 
  * Object based drawing program (i.e. MacDraw, not MacPaint). Lots of Edison progress, I know hundreds of things that don’t work. (Anyone write one of these?)
  * An interesting DB and query language that had some novel temporal capabilities. 

Currently fighting with a “spreadsheet engine”, though I’m teetering on the abyss of machine learning and might step off.

If it needs graphics or a GUI, it’s Java. If not I tend towards Common Lisp or Elisp.