I wrote a python script that runs in the background of my home PC all the time and checks every keystroke ever, so that it can pop up a Tk textbox if I press Ctrl twice, which then sends the input to a Google search. That's because I don't want to keep a browser open. I often forget to kill that process when gaming.
I wrote a C# WPF app to categorize the ~70 members of my church community into 3-5 person groups, for our twice-a-year weekend spiritual retreat, randomly but based on a set of constraints. It runs a naive first fit graph coloring algorithm. You can specify constraints on the UI that would make it run forever, so I added a Cancel button. The app exports an Excel file which contains a lot of ugly VBA macros written by a previous member of the community. The macros generate our name badges.
I started writing a declarative GUI library for Python Tkinter and used it for my thesis.
Actually now I realize that the absurd things I've built so far are the ones I'm proud of.