Several over the years, but most were "disposable". One I made and have used for something like 15 years is a Windows tray app - I click it and the cursor turn

Several over the years, but most were "disposable". One I made and have used for something like 15 years is a Windows tray app - I click it and the cursor turns to a dropper that let's me select any pixel on screen, then it tells me the RGB, HSL and hex code for the colour of the pixel. I'm colour-blind, so this is frequently useful.