I discovered how to do more neat effects in PSP. This probably doesn't sound like much, but for years I knew the really basic stuff: crop, resize... flood fill... really stupidly basic stuff. And then in the past month I learned how to do brushes, transparencies, erase (! yes, I never understood it before) and more. Wow.
This is like opening the door in the back of the wardrobe, I guess. ;D