GIMP is the only software whose UI makes me feel completely stupid. Every 1-2 years, I install it, fire it up to crop an image. I stare at the screen, right click a few times, then I close the app after admitting total defeat. It's been like this for 20 years.
A few years ago, I found out that Irfanview runs under Wine. It's a great feeling to understand the UI of a program.